Summary of Significant Accounting Policies - 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
Numerator:
Net income									$ 4,847	$ 2,160	$ 3,554
Less: Preferred stock dividends									169	148	116
Net income applicable to common shareholders	$ 1,707	$ 889	$ 821	$ 1,261	$ (585)	$ 942	$ 678	$ 977	$ 4,678	$ 2,012	$ 3,438
Denominator:
Weighted average common shares outstanding (in shares)									328.2	347.8	362.0
Effect of dilutive potential common shares:
Stock options (in shares)									3.2	3.6	4.3
Restricted stock units(non-participating) and performance stock awards (in shares)									2.1	1.8	1.5
Weighted average common and dilutive potential common shares outstanding (in shares)									333.5	353.2	367.8
Earnings per common share - Basic (in dollars per share)	$ 5.32	$ 2.71	$ 2.47	$ 3.79	$ (1.71)	$ 2.72	$ 1.94	$ 2.76	$ 14.25	$ 5.78	$ 9.50
Earnings per common share - Diluted (in dollars per share)	$ 5.23	$ 2.67	$ 2.44	$ 3.74	$ (1.71)	$ 2.68	$ 1.91	$ 2.71	$ 14.03	$ 5.70	$ 9.35
Other Earnings Per Share Disclosures
Antidilutive stock options, exercise price exceeds market price (in shares)									3.7	2.0	1.5