Basis of Presentation and Significant Accounting Policies and Practices - Earnings Per 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
Basic EPS:
Net income	$ 1,688	$ 1,353	$ 1,380	$ 1,174	$ 1,070	$ 1,425	$ 1,305	$ 1,217	$ 5,595	$ 5,017	$ 5,580
Income allocated to RSUs									(27)	(32)	(43)
Income allocated to common stock for basic EPS									$ 5,568	$ 4,985	$ 5,537
Weighted average number of shares outstanding, basic (in shares)									921	936	970
Basic earnings per common share (in dollars per share)	$ 1.83	$ 1.47	$ 1.50	$ 1.25	$ 1.14	$ 1.51	$ 1.38	$ 1.29	$ 6.05	$ 5.33	$ 5.71
Dilutive effect of stock compensation plans									12	16	20
Diluted EPS:
Net income	$ 1,688	$ 1,353	$ 1,380	$ 1,174	$ 1,070	$ 1,425	$ 1,305	$ 1,217	$ 5,595	$ 5,017	$ 5,580
Income allocated to RSUs									(27)	(31)	(42)
Income allocated to common stock for diluted EPS									$ 5,568	$ 4,986	$ 5,538
Weighted average number of shares outstanding, diluted (in shares)									933	952	990
Diluted earnings per common share (in dollars per share)	$ 1.80	$ 1.45	$ 1.48	$ 1.24	$ 1.12	$ 1.49	$ 1.36	$ 1.26	$ 5.97	$ 5.24	$ 5.59