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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income-basic and diluted	$ 102	$ 180	$ 403	$ 491
Denominator:
Basic weighted-average common shares outstanding (in shares)	164.0	164.6	164.3	164.5
Diluted weighted-average shares	165.9	166.8	166.1	166.5
Earnings per share:
Basic (in USD per share)	$ 0.62	$ 1.09	$ 2.45	$ 2.98
Diluted (in USD per share)	$ 0.61	$ 1.08	$ 2.42	$ 2.95
Number of anti-dilutive stock-based awards (in shares)	0.6	0.0	0.7	0.3
Stock Options
Denominator:
Effect of stock-based awards (in shares)	1.1	1.2	1.1	1.1
Nonvested shares
Denominator:
Effect of stock-based awards (in shares)	0.8	1.0	0.7	0.9