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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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