Medical Claims Payable (Reconciliation of Net Incurred Medical Claims to Benefit Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Net Incurred Medical Claims to Benefit Expense [Line Items]
Net incurred medical claims	$ 65,521.0	$ 59,908.2	$ 55,763.9
Quality Improvement And Other Claims Expense	1,313.4	1,208.7	1,091.0
Benefit expense	66,834.4	61,116.9	56,854.9
Commercial and Specialty Business Segment [Member]
Reconciliation of Net Incurred Medical Claims to Benefit Expense [Line Items]
Net incurred medical claims	27,330.8	26,318.2	27,468.6
Government Business Segment [Member]
Reconciliation of Net Incurred Medical Claims to Benefit Expense [Line Items]
Net incurred medical claims	$ 38,190.2	$ 33,590.0	$ 28,295.3