Medical Claims Payable (Incurred and Paid Claims Development, Net of Reinsurance) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Incurred And Paid Claims Development, Net Of Reinsurance [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	$ 187,296.9
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	179,943.4
Commercial and Specialty Business Segment [Member]
Incurred And Paid Claims Development, Net Of Reinsurance [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	84,225.7
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	81,480.0
Commercial and Specialty Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2014 and Prior [Member]
Incurred And Paid Claims Development, Net Of Reinsurance [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	30,121.8	$ 30,096.5	$ 30,576.8
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	30,095.6	29,988.9	27,797.9
Commercial and Specialty Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2015 [Member]
Incurred And Paid Claims Development, Net Of Reinsurance [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	26,306.6	26,798.5
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	26,265.1	24,145.7
Commercial and Specialty Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2016 [Member]
Incurred And Paid Claims Development, Net Of Reinsurance [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	27,797.3
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	25,119.3
Government Business Segment [Member]
Incurred And Paid Claims Development, Net Of Reinsurance [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	103,071.2
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	98,463.4
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2014 and Prior [Member]
Incurred And Paid Claims Development, Net Of Reinsurance [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	30,911.5	30,971.0	31,290.9
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	30,906.0	30,916.9	$ 27,976.0
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2015 [Member]
Incurred And Paid Claims Development, Net Of Reinsurance [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	33,585.6	33,909.9
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	33,520.2	$ 29,800.3
Government Business Segment [Member] | Short-duration Insurance Contracts, Claim Year 2016 [Member]
Incurred And Paid Claims Development, Net Of Reinsurance [Line Items]
Cumulative incurred claims and allocated claim adjustment expenses, net of reinsurance	38,574.1
Cumulative paid claims and allocated claim adjustment expenses, net of reinsurance	$ 34,037.2