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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income—basic and diluted	$ 553	$ 102	$ 739	$ 403
Denominator:
Basic weighted-average common shares outstanding (in shares)	162.7	164.0	163.3	164.3
Diluted weighted-average shares	164.0	165.9	164.7	166.1
Earnings per share:
Basic (in USD per share)	$ 3.40	$ 0.62	$ 4.53	$ 2.45
Diluted (in USD per share)	$ 3.38	$ 0.61	$ 4.49	$ 2.42
Number of anti-dilutive stock-based awards (in shares)	1.1	0.6	1.3	0.7
Stock Options
Denominator:
Effect of stock-based awards (in shares)	0.8	1.1	0.9	1.1
Nonvested shares
Denominator:
Effect of stock-based awards (in shares)	0.5	0.8	0.5	0.7