Income Taxes - Reconciliation of Beginning and Ending Amounts of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Balance at January 1	$ 205	$ 424	$ 604
Additions based on tax positions taken in the current year	19	14	19
Additions based on tax positions of prior years	36	4	113
Reductions based on tax positions of prior years	(78)	(147)	(274)
Reductions due to settlements with taxing authorities	(10)	(85)	(27)
Reductions due to lapses in statutes of limitation	(4)	(5)	(11)
Balance at December 31	$ 168	$ 205	$ 424