Income Taxes - Reconciliation to Nominal Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes [Abstract]
Nominal federal income tax rate	35.00%
Reconciliation of total taxes to nominal federal rate details [Abstract]
Tax expense at nominal rate	$ 1,164	$ 1,156	$ 761
Effect of undistributed foreign earnings	(67)	(74)	(42)
Health insurance industry tax	109	83	0
State income tax (net of federal income tax benefit)	42	25	27
Other	2	20	(48)
Total income taxes	$ 1,250	$ 1,210	$ 698