Earnings (Loss) Per Common Share - Computation of Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings
Net income	$ 273	$ 630
Less: Preferred stock dividends	5	5
Net income available to common stockholders	$ 268	$ 625
Shares
Weighted average common shares outstanding, basic	358.5	360.0
Dilutive effect of warrants [1]	0.0	1.4
Dilutive effect of stock-based awards under compensation plans	2.6	3.3
Weighted average common shares outstanding and dilutive potential common shares	361.1	364.7
Earnings Per Share, Basic [Abstract]
Net income available to common stockholders	$ 0.75	$ 1.74
Earnings Per Share, Diluted [Abstract]
Net income available to common stockholders	$ 0.74	$ 1.71