Income Taxes - Reconciliation to Nominal Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	$ 1,043	$ 1,164	$ 1,156
Effect of undistributed foreign earnings	(57)	(67)	(74)
Health insurance industry tax	108	109	83
State income tax (net of federal income tax benefit)	18	42	25
Other	24	2	20
Total income taxes	$ 1,136	$ 1,250	$ 1,210