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
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income (loss)—basic and diluted	$ 695	$ (31)
Denominator:
Basic weighted-average common shares outstanding (in shares)	163.0	164.0
Diluted weighted-average shares	164.6	164.0
Earnings per share:
Basic (in USD per share)	$ 4.27	$ (0.19)
Diluted (in USD per share)	$ 4.22	$ (0.19)
Number of anti-dilutive stock-based awards (in shares)	0.7	2.8
Stock Options
Denominator:
Effect of stock-based awards (in shares)	0.9	0.0
Nonvested shares
Denominator:
Effect of stock-based awards (in shares)	0.7	0.0