Intangible Assets (Intangible Assets - Additional Information) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Intangible Assets by Major Class
Estimated weighted-average life (in years)	4 years 1 month 6 days
Amortization of Intangible Assets	$ 2	$ 2	$ 4