Income Taxes - Schedule of Provision for Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Current, Federal	$ 1,067	$ 1,129	$ 1,259
Current, State	120	125	119
Current, Foreign	19	37	40
Deferred, Federal	423	75	(163)
Deferred, State	3	(5)	(27)
Deferred, Foreign	6	17	33
Provision for income taxes	$ 1,638	$ 1,378	$ 1,261