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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income—basic and diluted	$ 217	$ 100	$ 186	$ 301
Denominator:
Basic weighted-average common shares outstanding (in shares)	163.2	164.3	163.6	164.4
Diluted weighted-average shares	164.5	166.0	165.0	166.2
Earnings per share:
Basic (in USD per share)	$ 1.33	$ 0.61	$ 1.13	$ 1.83
Diluted (in USD per share)	$ 1.32	$ 0.60	$ 1.12	$ 1.81
Number of anti-dilutive stock-based awards (in shares)	1.3	0.6	1.3	0.7
Stock Options
Denominator:
Effect of stock-based awards (in shares)	0.8	1.0	0.9	1.1
Nonvested shares
Denominator:
Effect of stock-based awards (in shares)	0.5	0.7	0.5	0.7