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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income—basic and diluted	$ 248	$ 553	$ 1,371	$ 739
Denominator:
Basic weighted-average common shares outstanding (in shares)	163.3	162.7	163.2	163.3
Diluted weighted-average shares	165.6	164.0	165.1	164.7
Earnings per share:
Basic (in USD per share)	$ 1.51	$ 3.40	$ 8.40	$ 4.53
Diluted (in USD per share)	$ 1.49	$ 3.38	$ 8.30	$ 4.49
Number of anti-dilutive stock-based awards (in shares)	0.0	1.1	0.4	1.3
Stock Options
Denominator:
Effect of stock-based awards (in shares)	1.4	0.8	1.2	0.9
Nonvested shares
Denominator:
Effect of stock-based awards (in shares)	0.9	0.5	0.7	0.5