Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits
Balance at beginning of year	$ 42	$ 40
Additions for tax positions related to the current year	0	2
Additions for tax positions related to prior years	11	2
Settlements for tax positions	(1)	(2)
Settlements for tax positions	(1)	0
Balance at end of year	$ 51	$ 42