Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income from continuing operations	$ 3,003	$ 4,591	$ 4,558
Less: Net income attributable to noncontrolling interests	41	49	45
Preferred stock dividends	229	236	236
Preferred stock discount accretion and redemptions	4	4	4
Net income from continuing operations attributable to common shareholders	2,729	4,302	4,273
Less: Dividends and undistributed earnings allocated to nonvested restricted shares	13	18	18
Net income from continuing operations attributable to basic common shareholders	2,716	4,284	4,255
Net income from discontinued operations attributable to common shareholders	4,555	827	788
Less: Undistributed earnings allocated to nonvested restricted shares	22	3	3
Net income from continuing operations attributable to diluted common shareholders	2,716	4,284	4,255
Net income from discontinued operations attributable to basic common shareholders	$ 4,533	$ 824	$ 785
Basic weighted-average common shares outstanding (in shares)	427	447	467
Basic earnings (loss) per common share from continuing operations (in dollars per share)	$ 6.37	$ 9.59	$ 9.11
Basic earnings from discontinued operations (dollars per share)	10.62	1.84	1.68
Basic earnings per common share (dollars per share)	$ 16.99	$ 11.43	$ 10.79
Less: Impact of earnings per share dilution from discontinued operations	$ 2	$ 10	$ 9
Net income from discontinued operations attributable to diluted common shareholders	$ 4,531	$ 814	$ 776
Dilutive potential common shares		1	3
Diluted weighted-average common shares outstanding (in shares)	427	448	470
Diluted earnings (loss) per common share from continuing operations (in dollars per share)	$ 6.36	$ 9.57	$ 9.06
Diluted earnings from discontinued operations (dollars per share)	10.60	1.82	1.65
Diluted earnings per common share (dollars per share)	$ 16.96	$ 11.39	$ 10.71