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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income—basic and diluted	$ 428	$ 217	$ 1,123	$ 186
Denominator:
Basic weighted-average common shares outstanding (in shares)	163.3	163.2	163.1	163.6
Diluted weighted-average shares	165.2	164.5	164.9	165.0
Earnings per share:
Basic (in USD per share)	$ 2.62	$ 1.33	$ 6.89	$ 1.13
Diluted (in USD per share)	$ 2.59	$ 1.32	$ 6.81	$ 1.12
Number of anti-dilutive stock-based awards (in shares)	0.4	1.3	0.7	1.3
Stock Options
Denominator:
Effect of stock-based awards (in shares)	1.2	0.8	1.1	0.9
Nonvested shares
Denominator:
Effect of stock-based awards (in shares)	0.7	0.5	0.7	0.5