Investments, Debt and Equity Securities [Abstract] Amortized Cost Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract] Due in one year or less Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount Due after one year through five years Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount Held-to-maturity Securities Held-to-maturity Securities Estimated Fair Value Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value [Abstract] Due in one year or less Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value Due after one year through five years Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value Estimated Fair Value, Total Held-to-maturity Securities, Fair Value Debt Disclosure [Abstract] Schedule of Long-term Debt Instruments Schedule of Long-term Debt Instruments [Table] Long-term Debt, Type [Axis] Long-term Debt, Type [Axis] Long-term Debt, Type [Domain] Long-term Debt, Type [Domain] Convertible Notes Convertible Debt [Member] Senior Notes Senior Notes [Member] Credit Facility Revolving Credit Facility [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] 1.125% Convertible Notes Cash Convertible Senior Notes due 2020 [Member] Cash Convertible Senior Notes due 2020 [Member] 5.375% Notes Senior Notes Due 2022 [Member] Senior Notes Due 2022 [Member] 4.875% Notes 4.875% Senior Notes [Member] 4.875% Senior Notes [Member] 1.625% Convertible Notes Convertible Senior Notes Due 2044 [Member] Convertible Senior Notes Due 2044 [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Current portion of long-term debt: Debt, Current [Abstract] Debt net of unamortized discount Long-Term Debt, Current Maturities, Net Of Unamortized Premium (Discount) Long-Term Debt, Current Maturities, Net Of Unamortized Premium (Discount) Lease financing obligations Capital Lease Obligations, Current Debt issuance costs Debt Issuance Costs, Current, Net Current portion of long-term debt Long-term Debt and Capital Lease Obligations, Current Non-current portion of long-term debt: Long-term Debt, Excluding Current Maturities [Abstract] Debt net of unamortized debt discount Long-Term Debt, Noncurrent, Net Of Unamortized Debt Discount Long-Term Debt, Noncurrent, Net Of Unamortized Debt Discount Debt issuance costs Debt Issuance Costs, Noncurrent, Net Senior notes Long-term Debt, Excluding Current Maturities Lease financing obligations Capital Lease Obligations, Noncurrent Total debt Long-term Debt and Capital Lease Obligations, Including Current Maturities Percentage of contractual interest rate on Notes Debt Instrument, Interest Rate, Stated Percentage Fair Value Disclosures [Abstract] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Carrying Value Reported Value Measurement [Member] Fair Value Estimate of Fair Value Measurement [Member] Convertible Notes Credit Facility Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair value of debt Debt Instrument, Fair Value Disclosure Document and Entity Information [Abstract] -- None. No documentation exists for this element. -- Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Current Fiscal Year End Date Current Fiscal Year End Date Entity Filer Category Entity Filer Category Document Type Document Type Document Period End Date Document Period End Date Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Amendment Flag Amendment Flag Trading Symbol Trading Symbol Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Condensed Financial Information [Abstract] Condensed Financial Information [Abstract] Condensed Financial Statements [Table] Condensed Financial Statements [Table] Condensed Financial Statements, Captions [Line Items] Condensed Financial Statements, Captions [Line Items] Aggregate principal amount of notes outstanding Long-term Debt, Gross Commitments and Contingencies Disclosure [Abstract] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Restructuring and Related Activities [Abstract] Schedule of Restructuring and Related Costs [Table] Schedule of Restructuring and Related Costs [Table] Restructuring Plan [Axis] Restructuring Plan [Axis] Restructuring Plan [Domain] Restructuring Plan [Domain] 2017 Restructuring Plan 2017 Restructuring Plan [Member] 2017 Restructuring Plan [Member] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Health Plans Health Plans [Member] Health plans. Molina Medicaid Solutions Medicaid Solutions Segment [Member] Molina Medicaid solutions segment. Other Other Segments [Member] Restructuring Type [Axis] Restructuring Type [Axis] Type of Restructuring [Domain] Type of Restructuring [Domain] Separation Costs - Former Executives Employee Severance [Member] One-Time Termination Benefits One-time Termination Benefits [Member] Write-offs of Long-lived Assets Capitalized Software Write-Off [Member] Capitalized Software Write-Off [Member] Consulting Fees Professional Services Fees [Member] Professional Services Fees [Member] Contract Termination Costs Contract Termination [Member] Restructuring Cost and Reserve [Line Items] Restructuring Cost and Reserve [Line Items] Restructuring and separation costs Restructuring Costs Investments Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] Statement of Comprehensive Income [Abstract] Net income Net Income (Loss) Attributable to Parent Other comprehensive (loss) income: Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent [Abstract] Unrealized investment (loss) gain Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax Less: effect of income taxes Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Other comprehensive (loss) income, net of tax Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent Comprehensive income Comprehensive Income (Loss), Net of Tax, Attributable to Parent Segment Reporting [Abstract] Operating segment information Schedule of Segment Reporting Information, by Segment [Table Text Block] Amortized Cost Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract] Due in one year or less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis Due after one year through five years Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis Due after five years through ten years Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis Amortized Cost Available-for-sale Debt Securities, Amortized Cost Basis Estimated Fair Value Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract] Due in one year or less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value Due after one year through five years Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value Due after five years through ten years Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value Estimated Fair Value Available-for-sale Securities, Debt Maturities, without Single Maturity Date, Fair Value Accounting Policies [Abstract] Schedule of Segment Reporting Information, by Segment Schedule of Segment Reporting Information, by Segment [Table] Other segment Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Receivables: Accounts Receivable, Net Deferred contract costs (contract assets) – Molina Medicaid Solutions Contract with Customer, Asset, Net Deferred revenue (contract liabilities) – Molina Medicaid Solutions Contract with Customer, Liability Statement of Cash Flows [Abstract] Statement Statement [Table] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] 1.125% Call Option Call Option, Derivative Asset [Member] Call Option, Derivative Asset [Member] 1.125% Conversion Option Embedded Derivative Financial Instruments [Member] Statement [Line Items] Statement [Line Items] Operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation and amortization Depreciation, Depletion and Amortization Deferred income taxes Increase (Decrease) in Deferred Income Taxes Share-based compensation Share-based Compensation Non-cash restructuring costs Non-cash Restructuring Charges Non-cash Restructuring Charges Amortization of convertible senior notes and lease financing obligations Amortization Of Debt Discount (Premium) And Sale Leaseback Financing Obligation Amortization of Debt Discount (Premium) and Sale Leaseback Financing Obligation Loss on debt extinguishment Gain (Loss) on Extinguishment of Debt Other, net Other Operating Activities, Cash Flow Statement Changes in operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Receivables Increase (Decrease) in Receivables Prepaid expenses and other current assets Increase (Decrease) in Prepaid Expense and Other Assets Medical claims and benefits payable Increase (Decrease) in Health Care Insurance Liabilities Amounts due government agencies Increase (Decrease) In Medical Premium Liability Due To Agency Increase (Decrease) in Medical Premium Liability Due to Agency Accounts payable and accrued liabilities Increase (Decrease) in Accounts Payable and Accrued Liabilities Deferred revenue Increase (Decrease) in Deferred Revenue Income taxes Increase (Decrease) in Income Taxes Payable Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Purchases of investments Payments to Acquire Marketable Securities Proceeds from sales and maturities of investments Proceeds from Sale and Maturity of Marketable Securities Purchases of property, equipment and capitalized software Payments to Acquire Property, Plant, and Equipment Increase in restricted investments held-to-maturity Increase (Decrease) of Restricted Investments Other, net Payments for (Proceeds from) Other Investing Activities Net cash provided by (used in) investing activities Net Cash Provided by (Used in) Investing Activities Financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Cash paid for financing transaction fees Payments of Financing Costs Proceeds from employee stock plans Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options Other, net Proceeds from (Payments for) Other Financing Activities Net cash used in financing activities Net Cash Provided by (Used in) Financing Activities Net increase in cash, cash equivalents, and restricted cash and cash equivalents Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect Cash, cash equivalents, and restricted cash and cash equivalents at beginning of period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash, cash equivalents, and restricted cash and cash equivalents at end of period Supplemental cash flow information: Supplemental Cash Flow Information [Abstract] Schedule of non-cash investing and financing activities: Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Common stock used for share-based compensation Stock Issued Details of change in fair value of derivatives, net: Details of change in fair value of derivatives [Abstract] Details of change in fair value of derivatives Change in fair value of derivatives, net Derivative, Gain (Loss) on Derivative, Net 1.625% Convertible Notes exchange transaction: Business Combination, Description [Abstract] Common stock issued in exchange for 1.625% Convertible Notes Notes Issued Component of 1.625% Convertible Notes allocated to additional paid-in capital, net of income taxes Notes Issued, Fair Value Adjustment Increase (Decrease) Notes Issued, Fair Value Adjustment Increase (Decrease) Net increase to additional paid-in capital Adjustments to Additional Paid in Capital, Convertible Debt with Conversion Feature Separation Costs - Former Executives One-Time Termination Benefits Other Restructuring Costs Other Restructuring [Member] Restructuring Reserve Restructuring Reserve [Roll Forward] Accrued as of December 31, 2017 Restructuring Reserve Adjustments Restructuring Reserve, Accrual Adjustment Charges Restructuring Charges Cash payments Payments for Restructuring Accrued as of March 31, 2018 1.625% Convertible Notes Percentage of contractual interest rate on derivative Derivative, Fixed Interest Rate Percentage of contractual interest rate Earnings Per Share [Abstract] Summary of denominators for the computation of basic and diluted net income (loss) per share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Operating Segments Operating Segments [Member] Other Total revenue Revenues Restructuring and Separation Costs Restructuring and Related Activities Disclosure [Text Block] Restructuring Costs Restructuring and Related Costs [Table Text Block] Restructuring Reserve Schedule of Restructuring Reserve by Type of Cost [Table Text Block] Long term debt Schedule of Long-term Debt Instruments [Table Text Block] Interest Costs Schedule Of Debt Instruments Interest Cost Recognized [Table Text Block] Schedule of debt instruments interest cost recognized. Investments Schedule of Available-for-sale Securities Reconciliation [Table Text Block] Contractual maturities of investments Investments Classified by Contractual Maturity Date [Table Text Block] Schedule of available for sale securities continuous unrealized loss position Schedule Of Available For Sale Securities Continuous Unrealized Loss Position [Table Text Block] Schedule of available for sale securities continuous unrealized loss position. Other Liabilities Disclosure [Abstract] Non-risk provider payables Medical Claims And Benefits Payable Medical Claims and Benefits Payable Liability for Claims and Claims Adjustment Expense [Table] Liability for Claims and Claims Adjustment Expense [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Liability for Claims and Claims Adjustment Expense [Line Items] Liability for Claims and Claims Adjustment Expense [Line Items] Percentage of IBNP not expected to be paid Short-Duration Insurance Contracts, Liability For Unpaid Claims And Claims Adjustment Expense, Uncollectible Claims Assumption Short-Duration Insurance Contracts, Liability For Unpaid Claims And Claims Adjustment Expense, Uncollectible Claims Assumption Expense (recovery) for prior period claims development Prior Year Claims and Claims Adjustment Expense Schedule of liability for unpaid claims and claims adjustment expense Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block] Components of change in medical claims and benefits payable Components Of Change In Medical Claims And Benefits Payable [Table Text Block] Components Of Change In Medical Claims And Benefits Payable [Table Text Block] Fee-for-service claims incurred but not paid (IBNP) Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Amount Pharmacy payable Pharmacy Claims Payable Pharmacy Claims Payable Capitation payable Capitation Claims Payable Capitation Claims Payable Other Other Claims Payable Other Claims Payable Medical claims and benefits payable Liability for Claims and Claims Adjustment Expense Organization, Consolidation and Presentation of Financial Statements [Abstract] Basis of Presentation Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Stockholders' Equity Note [Abstract] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Class of Warrant or Right [Axis] Class of Warrant or Right [Axis] Class of Warrant or Right [Domain] Class of Warrant or Right [Domain] Cash Convertible Senior Notes Due 2020, Warrants Cash Convertible Senior Notes Due 2020, Warrants [Member] Cash Convertible Senior Notes Due 2020, Warrants [Member] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Common Stock Common Stock [Member] Award Type [Axis] Award Type [Axis] Equity Award [Domain] Equity Award [Domain] Restricted Stock Awards Restricted Stock [Member] Performance Stock Awards Performance Shares [Member] Performance Stock Units Restricted Stock Units (RSUs) [Member] Employee Stock Option Employee Stock Option [Member] Class of Stock [Line Items] Class of Stock [Line Items] Debt conversion (in shares) Debt Conversion, Converted Instrument, Shares Issued Number of warrants issued (in shares) Debt Conversion, Converted Instrument, Warrants or Options Issued Warrant, strike price per share (in dollars per share) Class of Warrant or Right, Exercise Price of Warrants or Rights Stated percentage of warrants Class Of Warrant Or Right, Stated Interest Rate Class Of Warrant Or Right, Stated Interest Rate Number of common stock issued (in shares) Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures Unrecognized compensation expense Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Weighted average period for recognition Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Unrecognized compensation forfeiture rate Unrecognized Compensation Forfeited Rate Unrecognized compensation forfeited rate. Stock options granted (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross Stock options exercised (in shares) Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period Equity [Abstract] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Total fair value of awards granted Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Granted In Period, Fair Value Share-Based Compensation Arrangement By Share-Based Payment Award, Equity Instruments Other Than Options, Granted In Period, Fair Value Total fair value of awards vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Restricted share activity Schedule of Nonvested Restricted Stock Units Activity [Table Text Block] Schedule of Deferred Compensation Arrangement with Individual, Share-based Payments [Table] Schedule of Deferred Compensation Arrangement with Individual, Share-based Payments [Table] Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items] Number of Shares Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Beginning Balance (shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Granted (shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Vested (shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Forfeited (shares) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Ending Balance (shares) Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Begining Balance (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Granted (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Vested (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Forfeited (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Ending Balance (usd per share) Schedule Of Premium Revenue By Health Plan Type Schedule of Premium Revenue by Health Plan Type [Table] Schedule of premium revenue by health plan type. Geographical [Axis] Geographical [Axis] Geographical [Domain] Geographical [Domain] Select Health Plans California, Illinois, New Mexico, Ohio, South Carolina, Texas, Washington, and Wisconsin [Member] California, Illinois, New Mexico, Ohio, South Carolina, Texas, Washington, and Wisconsin [Member] Schedule of Premium Revenue by Health Plan Type [Line Items] Schedule of Premium Revenue by Health Plan Type [Line Items] Schedule of premium revenue by health plan type. Percentage of additional incremental revenue earned Percentage Of Additional Incremental Revenue Earned Percentage of additional incremental revenue earned. Maximum available quality incentive premium - current period Maximum Available Quality Incentive Premium Current Year Maximum available quality incentive premium current year. Quality incentive premium revenue recognized in current period: Quality Incentive Premium Revenue Recognized [Abstract] Quality Incentive Premium Revenue Recognized [Abstract] Earned current period Amount Of Current Year Quality Incentive Premium Revenue Recognized Amount of current year quality incentive premium revenue recognized. Earned prior periods Amount Of Quality Incentive Premium Revenue Recognized From Prior Year Amount of quality incentive premium revenue recognized from prior year. Total Quality Incentive Premium Revenue Recognized Quality incentive premium revenue recognized. Quality incentive premium revenue recognized as a percentage of total premium revenue Quality Incentive Premium Revenue As A Percentage Of Total Revenue Quality Incentive Premium Revenue As A Percentage Of Total Revenue Statement of Stockholders' Equity [Abstract] Significant Accounting Policies Significant Accounting Policies [Text Block] Stockholders' Equity Stockholders' Equity Note Disclosure [Text Block] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Restricted cash and cash equivalents Restricted Cash and Cash Equivalents Total cash, cash equivalents, and restricted cash and cash equivalents presented in the statements of cash flows Medical Claims and Benefits Payable Medical Claims And Benefits Payable [Text Block] Medical claims and benefits payable. Other operating Segment Reconciling Items [Member] Gross margin Gross Profit Add: other operating revenues Less: other operating expenses Costs and Expenses Operating income Operating Income (Loss) Other expenses (income), net Nonoperating Income (Expense) Income before income tax expense Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Condensed Statement of Comprehensive Income [Table] Condensed Statement of Comprehensive Income [Table] Consolidated Entities [Axis] Consolidated Entities [Axis] Consolidated Entities [Domain] Consolidated Entities [Domain] Parent Guarantor Parent Company [Member] Other Guarantors Guarantor Subsidiaries [Member] Non-Guarantors Non-Guarantor Subsidiaries [Member] Reportable Legal Entities Reportable Legal Entities [Member] Eliminations Consolidation, Eliminations [Member] Condensed Statement of Income Captions [Line Items] Condensed Statement of Income Captions [Line Items] Other comprehensive income, net of tax Restricted Cash and Cash Equivalents Restrictions on Cash and Cash Equivalents [Table Text Block] Schedule of Cash and Cash Equivalents Schedule of Cash and Cash Equivalents [Table Text Block] Quality incentive premium revenue recognized Quality Incentive Premium Revenue Recognized [Table Text Block] Quality incentive premium revenue recognized. Segment reporting Number of reportable segments (in segment) Number of Reportable Segments Condensed Consolidating Statements of Income Condensed Income Statement [Table Text Block] Condensed Consolidating Statements of Comprehensive Income Condensed Statement of Comprehensive Income [Table Text Block] Condensed Consolidating Balance Sheets Condensed Balance Sheet [Table Text Block] Condensed Consolidating Statements of Cash Flows Condensed Cash Flow Statement [Table Text Block] Schedule of Available-for-sale Securities Schedule of Available-for-sale Securities [Table] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Axis] Major Types of Debt and Equity Securities [Domain] Major Types of Debt and Equity Securities [Domain] Corporate debt securities Corporate Debt Securities [Member] U.S. treasury notes US Treasury Notes Securities [Member] GSEs US Government-sponsored Enterprises Debt Securities [Member] Municipal securities Municipal Securities [Member] Municipal securities. Asset-backed securities Asset-backed Securities [Member] Schedule of Available-for-sale Securities [Line Items] Schedule of Available-for-sale Securities [Line Items] In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value In a Continuous Loss Position for Less than 12 Months, Unrealized Losses Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss In a Continuous Loss Position for Less than 12 Months, Number of Positions (in security) Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year In a Continuous Loss Position for 12 Months or More, Estimated Fair value Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value In a Continuous Loss Position for 12 Months or More, Unrealized Losses Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss In a Continuous Loss Position for 12 Months or More, Number of Positions (in security) Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year Contractual interest at coupon rate Interest Expense, Debt, Excluding Amortization Amortization of the discount Amortization of Debt Discount (Premium) Interest expense, debt Interest Expense, Debt Condensed Cash Flow Statement [Table] Condensed Cash Flow Statement [Table] Condensed Cash Flow Statements, Captions [Line Items] Condensed Cash Flow Statements, Captions [Line Items] Net cash provided by operating activities Decrease in restricted investments held-to-maturity Capital contributions to subsidiaries Payments to Acquire Additional Interest in Subsidiaries Dividends from subsidiaries Proceeds from Dividends Received Change in amounts due to/from affiliates Changes In Amounts Due To And Due From Affiliates Changes in amounts due to and due from affiliates. Net increase in cash, cash equivalents, and restricted cash and cash equivalents Derivative Instruments and Hedging Activities Disclosure [Abstract] Derivatives Derivative Instruments and Hedging Activities Disclosure [Text Block] Net Income per Share Earnings Per Share [Text Block] Certificate of deposit Certificates of Deposit [Member] Other Other Financial Instruments [Member] Other Financial Instruments [Member] Subtotal - current investments Current Investments [Member] Current Investments [Member] Restricted Cash and Cash Equivalents [Axis] Restricted Cash and Cash Equivalents [Axis] Cash and Cash Equivalents [Domain] Cash and Cash Equivalents [Domain] U.S. treasury notes US Treasury Securities [Member] Subtotal - current restricted investments Restricted Investments, Current [Member] Restricted Investments, Current [Member] Gross Unrealized Gains Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax Gross Unrealized Losses Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax Estimated Fair Value Available-for-sale Securities Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] CMS Subsidies CMS Subsidies [Member] CMS Subsidies [Member] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Medical Premium Liability Due to Agency Medical Premium Liability Due to Agency [Member] Medical Premium Liability Due to Agency [Member] Products and Services [Axis] Products and Services [Axis] Products and Services [Domain] Products and Services [Domain] Medicaid Expansion Medicaid Expansion [Member] Medicaid Expansion [Member] Service revenue relating to unsatisfied performance obligations Revenue, Remaining Performance Obligation Medical premiums liability based on medical cost thresholds Medical Premiums Liability, Medical Care Costs Threshold Medical Premiums Liability, Medical Care Costs Threshold Amounts due government agencies Medical Premium Liability Due To Agency Medical Premium Liability Due to Agency Risk adjustment receivable (payable) Affordable Care Act, Premium Stabilization Program, Risk Adjustment, Receivable (Payable) Affordable Care Act, Premium Stabilization Program, Risk Adjustment, Receivable (Payable) Operating expense Disposal Group, Including Discontinued Operation, Operating Expense Statement of Financial Position [Abstract] Common stock, par value (usd per share) Common Stock, Par or Stated Value Per Share Common stock, shares authorized (in shares) Common Stock, Shares Authorized Common stock, shares outstanding (in shares) Common Stock, Shares, Outstanding Preferred stock, par value (usd per share) Preferred Stock, Par or Stated Value Per Share Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding Fair Value Measurements Fair Value Disclosures [Text Block] Schedule of Derivative Instruments in Statement of Financial Position, Fair Value Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Additional Paid-in Capital Additional Paid-in Capital [Member] Accumulated Other Comprehensive Loss AOCI Attributable to Parent [Member] Retained Earnings Retained Earnings [Member] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Topic 606 Accounting Standards Update 2014-09 [Member] ASU 2018-02 Accounting Standards Update 2018-02 [Member] Accounting Standards Update 2018-02 [Member] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Beginning Balance (in shares) Shares, Outstanding Beginning Balance Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Adoption of ASU Cumulative Effect of New Accounting Principle in Period of Adoption 1.625% Convertible Notes exchange transaction (in shares) 1.625% Convertible Notes exchange transaction Other comprehensive loss, net Share-based compensation Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures Ending Balance (in shares) Ending Balance Derivative Derivative [Table] Current assets: Derivative asset Current Assets [Member] Current Assets [Member] Current liabilities: Derivative liability Current Liabilities [Member] Current Liabilities [Member] 1.125% Conversion Option derivative liability Derivative [Line Items] Derivative [Line Items] Derivative assets Derivative Asset, Fair Value, Gross Asset Derivative liabilities Derivative Liability, Fair Value, Gross Liability Percentage of contractual interest rate on Call Option Fair value of assets measured on recurring basis Fair Value, Assets Measured on Recurring Basis [Table Text Block] Schedule of fair value, asset and liabilities measured on recurring basis - disclosure only Schedule Of Fair Value, Asset And Liabilities Measured On Recurring Basis [Table Text Block] Schedule of Fair Value, Asset and Liabilities Measured on Recurring Basis [Table Text Block] Current assets: Assets, Current [Abstract] Investments Marketable Securities, Current Restricted investments Restricted Investments, Current Receivables Receivables, Net, Current Prepaid expenses and other current assets Prepaid Expense and Other Assets, Current Derivative asset Derivative Asset, Current Total current assets Assets, Current Property, equipment, and capitalized software, net Property, Plant and Equipment, Net Goodwill and intangible assets, net Intangible Assets, Net (Including Goodwill) Restricted investments Restricted Investments, Noncurrent Deferred income taxes Deferred Income Tax Assets, Net Other assets Other Assets, Noncurrent Total assets Assets Current liabilities: Liabilities, Current [Abstract] Medical claims and benefits payable Accounts payable and accrued liabilities Accounts Payable and Accrued Liabilities, Current Deferred revenue Deferred Revenue, Current Current portion of long-term debt Derivative liability Derivative Liability, Current Total current liabilities Liabilities, Current Long-term debt Other long-term liabilities Other Liabilities, Noncurrent Total liabilities Liabilities Stockholders’ equity: Stockholders' Equity Attributable to Parent [Abstract] Common stock, $0.001 par value, 150 shares authorized; outstanding: 62 shares at March 31, 2018 and 60 shares at December 31, 2017 Common Stock, Value, Outstanding Preferred stock, $0.001 par value; 20 shares authorized, no shares issued and outstanding Preferred Stock, Value, Issued Additional paid-in capital Additional Paid in Capital, Common Stock Accumulated other comprehensive loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained earnings Retained Earnings (Accumulated Deficit) Total stockholders’ equity Stockholders' Equity Attributable to Parent Total liabilities and stockholders' equity Liabilities and Equity Presentation and Reclassification Reclassification, Policy [Policy Text Block] Consolidation And Interim Financial Information Consolidation And Interim Financial Information, Policy [Policy Text Block] Consolidation And Interim Financial Information, Policy [Policy Text Block] Cash and Cash Equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Revenue Recognition Revenue Recognition, Policy [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Recent Accounting Pronouncements New Accounting Pronouncements, Policy [Policy Text Block] Basis Of Presentation [Table] Basis Of Presentation [Table] Basis Of Presentation [Table] Basis Of Presentation [Line Items] Basis Of Presentation [Line Items] Basis Of Presentation [Line Items] Number of states in which entity operates (in state) Number of States in which Entity Operates Number of members eligible for the health care programs, approximately (in member) Number Of Members Eligible For The Health Care Programs Number of members eligible for the health care programs. Minimum contract terms Minimum Contract Term Minimum contract term. Maximum contract terms Maximum Contract Term Maximum contract term. Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Numerator: Net Income (Loss) Available to Common Stockholders, Basic [Abstract] Denominator: Weighted Average Number of Shares Outstanding Reconciliation [Abstract] Shares outstanding at the beginning of the period (in shares) 1.625% Exchange (in shares) Weighted Average Number of Shares Issued, Basic Denominator for basic net income per share (in shares) Weighted Average Number of Shares Outstanding, Basic Effect of dilutive securities: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Convertible senior notes (in shares) Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities 1.125% Warrants (in shares) Incremental Common Shares Attributable to Dilutive Effect of Call Options and Warrants Denominator for diluted net income per share (in shares) Weighted Average Number of Shares Outstanding, Diluted Net income per share: Earnings Per Share, Basic and Diluted [Abstract] Basic (in dollars per share) Earnings Per Share, Basic Diluted (in dollars per share) Earnings Per Share, Diluted Segments Segment Reporting Disclosure [Text Block] Line of Credit Line of Credit [Member] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Letter of Credit Letter of Credit [Member] Bridge Credit Agreement Bridge Credit Agreement [Member] Bridge Credit Agreement [Member] Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Term of debt instrument Debt Instrument, Term Amount outstanding under the Credit Facility Line of Credit, Current Amount outstanding under Letter of Credit Long-term Line of Credit Current borrowing capacity Line of Credit Facility, Remaining Borrowing Capacity Face amount Debt Instrument, Face Amount Acquisition financings Debt Instrument, Proceeds Used for Working Capital and Acquisition Financings Debt Instrument, Proceeds Used for Working Capital and Acquisition Financings Future reduction to face amount Debt Instrument, Future Reduction to Face Amount Debt Instrument, Future Reduction to Face Amount Restricted investments transferred to unrestricted investments during period Restricted Investments Transferred To Unrestricted During Period Restricted Investments Transferred To Unrestricted During Period Conversion ratio Debt Instrument, Convertible, Conversion Ratio Conversion price per share of common stock (in dollars per share) Debt Instrument, Convertible, Conversion Price Stock price trigger (in dollars per share) Debt Instrument, Convertible, Stock Price Trigger Effective interest percentage Debt Instrument, Interest Rate, Effective Percentage Remaining amortization period Debt Instrument, Convertible, Remaining Discount Amortization Period If-converted value Debt Instrument, Convertible, If-converted Value in Excess of Principal Debt conversion Debt Conversion, Converted Instrument, Amount Loss on debt extinguishment Discount amortization period Debt Instrument, Convertible, Discount Amortization Period Debt Instrument, Convertible, Discount Amortization Period Carrying amount of equity component Debt Instrument, Convertible, Carrying Amount of Equity Component Supplemental Condensed Consolidating Financial Information Condensed Financial Statements [Text Block] Income Statement [Abstract] Revenue: Revenues [Abstract] Premium revenue Health Care Organization, Premium Revenue Service revenue Technology Services Revenue Premium tax revenue Health Care Organization, Premium Tax Revenue Health Care Organization, Premium Tax Revenue Health insurer fees reimbursed Health Care Organization, Insurer Fee Revenue Health Care Organization, Insurer Fee Revenue Investment income and other revenue Interest and Dividend Income, Operating Total revenue Operating expenses: Costs and Expenses [Abstract] Medical care costs Policyholder Benefits and Claims Incurred, Net, Health Cost of service revenue Technology Services Costs General and administrative expenses General and Administrative Expense Premium tax expenses Premium Tax Expenses An assessment levied by a state government on the net premium income collected. Health insurer fees Health Care Organization, Insurer Fee Expense Health Care Organization, Insurer Fee Expense Depreciation and amortization Depreciation, Depletion and Amortization, Nonproduction Restructuring and separation costs Total operating expenses Operating income Other expenses (income), net: Nonoperating Income (Expense) [Abstract] Interest expense Interest Expense Other expense (income), net Other Nonoperating Income (Expense) Total other expenses (income), net Income tax expense Income Tax Expense (Benefit) Net income Condensed Balance Sheet Statement [Table] Condensed Balance Sheet Statement [Table] Condensed Balance Sheet Statements, Captions [Line Items] Condensed Balance Sheet Statements, Captions [Line Items] ASSETS Assets [Abstract] Due from (to) affiliates Related Party Transaction, Due from (to) Related Party, Current Investment in subsidiaries, net Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures LIABILITIES AND STOCKHOLDERS’ EQUITY Liabilities and Equity [Abstract] Long-term debt Long-term Debt and Capital Lease Obligations Deferred income taxes Deferred Income Tax Liabilities, Net Total stockholders’ equity Insurance Claims Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward] Medical claims and benefits payable, beginning balance Components of medical care costs related to: Components Of Medical Care Costs [Abstract] Components of medical care costs. Current period Current Year Claims and Claims Adjustment Expense Prior periods Total medical care costs Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims Change in non-risk provider payables Medical Claims And Benefits Payable, Adjustments, Change In Provider Medical Claims and Benefits Payable, Adjustments, Change in Provider Payments for medical care costs related to: Payments For Medical Care Costs [Abstract] Payments for medical care costs. Current period Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year Prior periods Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years Total paid Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid Medical claims and benefits payable, ending balance Benefit from prior period as a percentage of: Benefit From Prior Years As Percentage [Abstract] Benefit from prior years as percentage. Balance at beginning of period Policy Holder Benefits Prior Percentage Policy holder benefits prior percentage. Premium revenue, trailing twelve months Policy Holder Premium Revenue Prior Year Percentage Policy holder premium revenue prior year percentage. Medical care costs, trailing twelve months Policy Holder Medical Care Cost Prior Year Percentage Policy holder medical care cost prior year percentage. Fair Value Measurements, Recurring and Nonrecurring Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Recurring Fair Value, Measurements, Recurring [Member] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Quoted Market Prices (Level 1) Fair Value, Inputs, Level 1 [Member] Significant Other Observable Inputs (Level 2) Fair Value, Inputs, Level 2 [Member] Significant Unobservable Inputs (Level 3) Fair Value, Inputs, Level 3 [Member] Eligible Item or Group for Fair Value Option [Axis] Financial Instrument [Axis] Fair Value, Option, Eligible Item or Group [Domain] Financial Instruments [Domain] U.S. treasury notes Government-sponsored enterprise securities (GSEs) 1.125% Call Option derivative asset Derivative Financial Instruments, Assets [Member] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Total assets measured at fair value on a recurring basis Assets, Fair Value Disclosure 1.125% Conversion Option derivative liability Derivative Liability Total liability measured at fair value on a recurring basis Financial and Nonfinancial Liabilities, Fair Value Disclosure Condensed Income Statement [Table] Condensed Income Statement [Table] Condensed Income Statements, Captions [Line Items] Condensed Income Statements, Captions [Line Items] Expenses: Restructuring and separation costs Net income (losses) before equity in earnings (losses) of subsidiaries Net Income (Loss) From Continuing Operations Before Equity Method Investments, Extraordinary Items, Noncontrolling Interest Net Income (Loss) From Continuing Operations Before Equity Method Investments, Extraordinary Items, Noncontrolling Interest Equity in net earnings of subsidiaries Income (Loss) from Subsidiaries, Net of Tax Debt Long-term Debt [Text Block] Entities [Table] Entities [Table] Parent Company Entity Information [Line Items] Entity Information [Line Items] Net assets of subsidiaries subject to restrictions Amount of Restricted Net Assets for Consolidated and Unconsolidated Subsidiaries Cash, cash equivalents, and investments (excluding restricted investments) Cash, Cash Equivalents, and Short-term Investments Aggregate statutory capital and surplus Statutory Accounting Practices, Statutory Capital and Surplus, Balance Required minimum statutory capital surplus Statutory Accounting Practices, Statutory Capital and Surplus Required