Medical Claims Payable (Incurred and Paid Claims Development, Net of Reinsurance) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	$ 239,548
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	228,459
Commercial and Specialty Business [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	77,213
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	73,932
Commercial and Specialty Business [Member] | Short-duration Insurance Contracts, Claim Year 2018 and Prior [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	26,747	$ 26,753	$ 26,943
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	26,721	26,643	24,367
Commercial and Specialty Business [Member] | Short-duration Insurance Contracts, Claim Year 2019 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	25,572	25,942
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	25,475	23,026
Commercial and Specialty Business [Member] | Short-Duration Insurance Contract, Claim Year 2020 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	24,894
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	21,736
Government Business Segment [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	160,849
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	153,236
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2018 and Prior [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	49,278	49,306	49,616
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	49,231	49,201	44,959
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2019 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	52,518	52,753
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	52,403	47,269
Government Business Segment [Member] | Short-Duration Insurance Contract, Claim Year 2020 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	59,053
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	51,602
Other Segment [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	1,486
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	1,291
Other Segment [Member] | Short-duration Insurance Contracts, Claim Year 2018 and Prior [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	0	0	0
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	0	0	$ 0
Other Segment [Member] | Short-duration Insurance Contracts, Claim Year 2019 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	0	0
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	0	$ 0
Other Segment [Member] | Short-Duration Insurance Contract, Claim Year 2020 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	1,486
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 1,291