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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net income-basic and diluted	$ 176	$ 84	$ 304	$ 175
Denominator:
Basic weighted-average common shares outstanding (in shares)	164.1	163.5	164.1	163.5
Diluted weighted-average shares	165.5	165.1	165.5	165.1
Earnings per share:
Basic (in USD per share)	$ 1.07	$ 0.51	$ 1.85	$ 1.07
Diluted (in USD per share)	$ 1.06	$ 0.51	$ 1.84	$ 1.06
Number of anti-dilutive stock-based awards (in shares)	0.7	0.7	0.7	0.7
Stock Options
Denominator:
Effect of stock-based awards (in shares)	0.9	1.0	0.9	1.0
Nonvested shares
Denominator:
Effect of stock-based awards (in shares)	0.5	0.6	0.5	0.6