Medical Claims Payable (Incurred and Paid Claims Development, Net of Reinsurance) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	$ 208,919
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	201,687
Commercial and Specialty Business [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	82,504
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	79,928
Commercial and Specialty Business [Member] | Short-duration Insurance Contracts, Claim Year 2018 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	24,094
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	21,633
Commercial and Specialty Business [Member] | Short-duration Insurance Contracts, Claim Year 2017 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	29,112	$ 29,467
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	29,006	26,250
Commercial and Specialty Business [Member] | Short-duration Insurance Contracts, Claim Year 2016 and Prior [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	29,298	29,399	$ 29,861
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	29,289	29,311	27,135
Government Business Segment [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	126,415
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	121,759
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2018 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	45,686
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	41,115
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2017 [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	40,596	40,986
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	40,541	36,673
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2016 and Prior [Member]
Claims Development [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	40,133	40,217	40,888
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 40,103	$ 40,126	$ 36,497