Income Taxes - Income Tax Expense (Benefit) Table (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Tax Disclosure [Abstract]
Current income tax expense (benefit)	$ 90	$ 762
Deferred income tax expense (benefit)	36	(665)
Total income tax expense (benefit)	126	97
Income (loss) before income taxes	$ 300	$ (178)
Effective tax rate	42.00%	(54.00%)