Medical Claims Payable (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	$ (500)	$ (930)	$ (1,133)
Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	(190)	(456)	(462)
Government Business Segment [Member]
Claims Development [Line Items]
Net incurred medical claims in prior years redundancies	(310)	$ (474)	$ (671)
Short-duration Insurance Contracts, Claim Year 2017 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	$ 31
Short-duration Insurance Contract, Cumulative Number of Reported Claims	121
Short-duration Insurance Contracts, Claim Year 2017 and Prior [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 16
Short-duration Insurance Contract, Cumulative Number of Reported Claims	221
Short-duration Insurance Contracts, Claim Year 2018 [Member] | Commercial and Specialty Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 78
Short-duration Insurance Contract, Cumulative Number of Reported Claims	89
Short-duration Insurance Contracts, Claim Year 2018 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 89
Short-duration Insurance Contract, Cumulative Number of Reported Claims	216
Short-Duration Insurance Contract, 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	$ 2,916
Short-duration Insurance Contract, Cumulative Number of Reported Claims	84
Short-Duration Insurance Contract, Claim Year 2019 [Member] | Government Business Segment [Member]
Claims Development [Line Items]
Short-duration Insurance Contracts, Incurred but Not Reported (IBNR) Claims Liability, Net	$ 5,484
Short-duration Insurance Contract, Cumulative Number of Reported Claims	230