Income Taxes - Income Tax Expense (Benefit) Table (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Current income tax expense (benefit)	$ 146	$ 430	$ 383	$ 670
Deferred income tax expense (benefit)	110	(855)	124	(1,036)
Total income tax expense (benefit)	256	(425)	507	(366)
Income (loss) before income taxes	$ 683	$ (1,066)	$ 1,125	$ (1,616)
Effective tax rate	37.00%	40.00%	45.00%	23.00%