Medical Claims Payable (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	$ (637)	$ (500)	$ (930)
Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	(375)	(190)	(456)
Government Business Segment [Member]
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	(262)	$ (310)	$ (474)
Other Segment [Member]
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	0
Short-duration Insurance Contracts, Claim Year 2018 and Prior [Member] | Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 26
Short-duration Insurance Contract, Cumulative Number of Reported Claims	93
Short-duration Insurance Contracts, Claim Year 2018 and Prior [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 47
Short-duration Insurance Contract, Cumulative Number of Reported Claims	225
Short-duration Insurance Contracts, Claim Year 2018 and Prior [Member] | Other Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contract, Cumulative Number of Reported Claims	0
Short-duration Insurance Contracts, Claim Year 2019 [Member] | Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 97
Short-duration Insurance Contract, Cumulative Number of Reported Claims	87
Short-duration Insurance Contracts, Claim Year 2019 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 115
Short-duration Insurance Contract, Cumulative Number of Reported Claims	241
Short-duration Insurance Contracts, Claim Year 2019 [Member] | Other Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 0
Short-duration Insurance Contract, Cumulative Number of Reported Claims	0
Short-Duration Insurance Contract, Claim Year 2020 [Member] | Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 3,158
Short-duration Insurance Contract, Cumulative Number of Reported Claims	74
Short-Duration Insurance Contract, Claim Year 2020 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 7,451
Short-duration Insurance Contract, Cumulative Number of Reported Claims	236
Short-Duration Insurance Contract, Claim Year 2020 [Member] | Other Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 195
Short-duration Insurance Contract, Cumulative Number of Reported Claims	27