Income Taxes - Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Balance beginning	$ 82	$ 71	$ 42
Additions based on tax positions related to the current year	19	19	18
Additions based on tax positions of prior years	11	4	21
Accrued interest	4	2	2
Reductions for tax positions of prior years		(7)	(1)
Settlements	(1)		(3)
Lapse of statute of limitations	(6)	(7)	(8)
Balance ending	109	82	71
Net unrecognized tax benefits that would impact effective tax rate	94
Accrued interest	7	5
Accrual for unrecognized tax benefits	$ 0	$ 0	$ 0