Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefit beginning balance	$ 1
Gross increases - tax positions in current period		$ 1
Settlement	(1)
Unrecognized tax benefit ending balance	$ 0	$ 1