Earnings Per Common Share - Computation of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings
Income (loss) from continuing operations, net of tax									$ 2,085	$ 1,485	$ (262)
Preferred Stock Dividends, Income Statement Impact									21	6	0
Income loss from Continuing Operations Net of Tax Available to Common Shareholders									2,064	1,479	(262)
Income (loss) from discontinued operations, net of tax, available to common stockholders									0	322	(2,869)
Net income (loss) available to common stockholders	$ 543	$ 524	$ 372	$ 625	$ 190	$ 432	$ 582	$ 597	$ 2,064	$ 1,801	$ (3,131)
Shares
Weighted average common shares outstanding, basic									360.9	358.4	363.7
Dilutive effect of warrants [1]									0.5	1.9	0.0
Dilutive effect of stock-based awards under compensation plans									3.5	3.8	0.0
Weighted average common shares outstanding and dilutive potential common shares [2]									364.9	364.1	363.7
Basic
Income (loss) from continuing operations, net of tax, available to common stockholders	$ 1.51	$ 1.45	$ 1.03	$ 1.74	$ 0.53	$ 1.19	$ 1.21	$ 1.20	$ 5.72	$ 4.13	$ (0.72)
Income (loss) from discontinued operations, net of tax, available to common stockholders	0	0	0	0	0	0.01	0.41	0.47	0	0.90	(7.89)
Net income (loss) available to common stockholders	1.51	1.45	1.03	1.74	0.53	1.20	1.62	1.67	5.72	5.03	(8.61)
Diluted
Income (loss) from continuing operations, net of tax, available to common stockholders	1.49	1.43	1.02	1.71	0.52	1.17	1.19	1.18	5.66	4.06	(0.72)
Income (loss) from discontinued operations, net of tax, available to common stockholders	0	0	0	0	0	0.02	0.41	0.46	0	0.89	(7.89)
Net income (loss) available to common stockholders	$ 1.49	$ 1.43	$ 1.02	$ 1.71	$ 0.52	$ 1.19	$ 1.60	$ 1.64	$ 5.66	$ 4.95	$ (8.61)
Stock Compensation Plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)											4.3
Warrant
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)											2.5