Income Taxes - Schedule of Activity Related to Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Beginning Balance	$ 377	$ 487
Additions based on tax positions related to the current year	40	11
Additions for tax positions of prior years	11	8
Reductions for tax positions of prior years	(13)	(18)
Settlements		(101)
Lapse of applicable statutes of limitations	(16)	(10)
Ending Balance	$ 399	$ 377