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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings
Net income	$ 459	$ 535	$ 1,200	$ 1,537
Less: Preferred stock dividends	6	11	16	16
Net income available to common stockholders	$ 453	$ 524	$ 1,184	$ 1,521
Shares
Weighted average common shares outstanding, basic	358.3	361.4	358.3	361.0
Dilutive effect of warrants [1]	0.0	0.0	0.0	0.7
Dilutive effect of stock-based awards under compensation plans	2.2	4.0	2.0	3.4
Weighted average common shares outstanding and dilutive potential common shares	360.5	365.4	360.3	365.1
Earnings Per Share, Basic [Abstract]
Net income available to common stockholders	$ 1.26	$ 1.45	$ 3.30	$ 4.21
Earnings Per Share, Diluted [Abstract]
Net income available to common stockholders	$ 1.26	$ 1.43	$ 3.29	$ 4.17