Changes in Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Beginning Balance	$ 59	$ 56	$ 63
Additions based on tax positions related to the current year	6	3	6
Additions for tax positions for prior years	9	3	3
Reductions for tax positions for prior years	(10)	(3)	(14)
Settlements	0	0	(1)
Statute of limitations	(1)	0	(1)
Ending Balance	$ 63	$ 59	$ 56