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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings
Net income	$ 468	$ 372	$ 741	$ 1,002
Less: Preferred stock dividends	5	0	10	5
Net income available to common stockholders	$ 463	$ 372	$ 731	$ 997
Shares
Weighted average common shares outstanding, basic	358.1	361.4	358.3	360.7
Dilutive effect of warrants [1]	0.0	0.5	0.0	0.9
Dilutive effect of stock-based awards under compensation plans	1.2	3.2	1.9	3.3
Weighted average common shares outstanding and dilutive potential common shares	359.3	365.1	360.2	364.9
Earnings Per Share, Basic [Abstract]
Net income available to common stockholders	$ 1.29	$ 1.03	$ 2.04	$ 2.76
Earnings Per Share, Diluted [Abstract]
Net income available to common stockholders	$ 1.29	$ 1.02	$ 2.03	$ 2.73