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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income									$ 5,576	$ 4,847	$ 2,160
Less: Preferred stock dividends									115	169	148
Net income applicable to common shareholders	$ 2,598	$ 1,126	$ 1,224	$ 513	$ 1,707	$ 889	$ 821	$ 1,261	$ 5,461	$ 4,678	$ 2,012
Denominator:
Weighted average common shares outstanding (in shares)									311.6	328.2	347.8
Effect of dilutive potential common shares:
Stock options (in shares)									2.2	3.2	3.6
Restricted stock units (non-participating) and performance stock awards (in shares)									1.7	2.1	1.8
Weighted average common and dilutive potential common shares outstanding (in shares)									315.5	333.5	353.2
Earnings per common share - Basic (in dollars per share)	$ 8.54	$ 3.62	$ 3.90	$ 1.62	$ 5.32	$ 2.71	$ 2.47	$ 3.79	$ 17.53	$ 14.25	$ 5.78
Earnings per common share - Diluted (in dollars per share)	$ 8.45	$ 3.58	$ 3.86	$ 1.59	$ 5.23	$ 2.67	$ 2.44	$ 3.74	$ 17.31	$ 14.03	$ 5.70
Other Earnings Per Share Disclosures
Antidilutive stock options, exercise price exceeds market price (in shares)									2.9	3.7	2.0