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Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jan. 01, 2019 | Dec. 31, 2015
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Final maturities of investments |  |  | 10 years |  |  |  |
Average maturity of investments |  |  | 3 years |  |  |  |
Amounts due government agencies | $ 967 |  | $ 967 | $ 1,542 |  |  |
Premium revenue |  |  | 17,612 | 18,854 | $ 16,445 |  |
Prepaid reinsurance premiums | 16 |  | 16 | 20 | 30 |  |
Reinsurance recoveries |  |  | 33 | 24 | 65 |  |
Reinsurance recoverable for unpaid claims adjustments | 31 |  | 31 | 16 | 61 |  |
Marketplace risk adjustment payable | (466) |  | (466) | (917) |  |  |
Net (decrease) increase in cash, cash equivalents, and restricted cash and cash equivalents | 2,926 |  | 2,926 | 3,290 | 2,912 |  | $ 2,426
Net cash provided by (used in) investing activities |  |  | 1,143 | (1,062) | (206) |  |
Medicaid Expansion |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Amounts due government agencies | 87 |  | 87 | 96 |  |  |
Medical Premium Liability Due to Agency |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Medical premium liability based on medical costs threshold | 103 |  | $ 103 | 135 |  |  |
Minimum |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Useful life |  |  | 5 years |  |  |  |
Minimum | California, Illinois, New Mexico, Ohio, South Carolina, Texas, Washington, and Wisconsin |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Percentage of additional incremental revenue earned |  |  | 1.00% |  |  |  |
Maximum |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Useful life |  |  | 15 years |  |  |  |
Maximum | California, Illinois, New Mexico, Ohio, South Carolina, Texas, Washington, and Wisconsin |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Percentage of additional incremental revenue earned |  |  | 3.00% |  |  |  |
CMS Subsidies |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Disposal group, operating expense |  |  | $ 81 |  |  |  |
California Department of Health Care Services |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Premium revenue | $ (24) | $ 57 |  |  |  |  |
Forecast |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Finance lease, liability |  |  |  |  |  | $ 230 |
Forecast | Minimum |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Operating lease, right-of-use asset |  |  |  |  |  | 80 |
Operating lease, liability |  |  |  |  |  | 90 |
Finance lease, right-of-use asset |  |  |  |  |  | 230 |
Forecast | Minimum | Property, Plant and Equipment |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Adoption of ASU |  |  |  |  |  | 75 |
Forecast | Minimum | Lease Financing Obligations |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Adoption of ASU |  |  |  |  |  | 195 |
Forecast | Minimum | Retained Earnings |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Adoption of ASU |  |  |  |  |  | 80 |
Forecast | Maximum |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Operating lease, right-of-use asset |  |  |  |  |  | 90 |
Operating lease, liability |  |  |  |  |  | 100 |
Finance lease, right-of-use asset |  |  |  |  |  | 240 |
Finance lease, liability |  |  |  |  |  | 240 |
Forecast | Maximum | Property, Plant and Equipment |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Adoption of ASU |  |  |  |  |  | 95 |
Forecast | Maximum | Lease Financing Obligations |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Adoption of ASU |  |  |  |  |  | 205 |
Forecast | Maximum | Retained Earnings |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Adoption of ASU |  |  |  |  |  | $ 100 |
Accounting Standards Update 2016-18 |  |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |  |
Net (decrease) increase in cash, cash equivalents, and restricted cash and cash equivalents |  |  |  | 104 | 93 |  |
Net cash provided by (used in) investing activities |  |  |  | $ (104) | $ (93) |  |

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