Income Taxes - Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current taxes
U.S. income taxes	$ 974	$ 935	$ 1,076
Foreign income taxes	122	95	93
State income taxes	36	32	60
Total taxes, current	1,132	1,062	1,229
Deferred taxes (benefits)
U.S. Income taxes (benefits)	204	69	22
Foreign income taxes (benefits)	39	9	(6)
State income taxes (benefits)	(1)	(4)	5
Total taxes (benefits), deferred	242	74	21
Total taxes	$ 1,374	$ 1,136	$ 1,250