Income Taxes - Reconciliation to Nominal Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 1,262	$ 1,043	$ 1,164
Effect of U.S. tax reform legislation	232	0	0
Effect of undistributed foreign earnings	(70)	(57)	(67)
Health insurance industry tax	0	108	109
State income tax (net of federal income tax benefit)	23	18	42
Other	(73)	24	2
Total income taxes	$ 1,374	$ 1,136	$ 1,250