Income Taxes - Components of (Benefit) Provision for Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 21	$ 130	$ 102
State	2	16	15
Foreign	9	2	8
Total current	32	148	125
Deferred:
Federal	(2,373)	(60)	69
State		(7)	(74)
Foreign	(5)	1	(2)
Total deferred	(2,378)	(66)	(7)
Total (benefit) provision for income taxes	$ (2,346)	$ 82	$ 118