Net Income Per Common Share - Calculations for Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income (loss)—basic and diluted	$ (31)	$ 201
Denominator:
Basic weighted-average common shares outstanding (in shares)	164.0	164.6
Diluted weighted-average shares	164.0	166.5
Earnings per share:
Basic (in USD per share)	$ (0.19)	$ 1.22
Diluted (in USD per share)	$ (0.19)	$ 1.21
Number of anti-dilutive stock-based awards (in shares)	2.8	0.7
Stock Options
Denominator:
Effect of stock-based awards (in shares)	0.0	1.1
Nonvested shares
Denominator:
Effect of stock-based awards (in shares)	0.0	0.8