Stock-Based Compensation - Summary of Changes In Stock Options (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Stock Based Compensation [Abstract]
Weighted Average Exercise Price Per Share, Outstanding, Beginning of Period	$ 16	$ 16	$ 20
Outstanding Shares, Beginning of Period	0.7
Outstanding Shares, Exercised	(0.3)
Outstanding Shares, End of Period	0.4
Weighted Average Exercise Price Per Share, Exercised	$ 25
Weighted Average Exercise Price Per Share, Outstanding, End of Period	$ 16
Weighted Average Remaining Life, Outstanding, End of Period	2 years 4 months 24 days
Aggregate Intrinsic Value, Outstanding		$ 6.3