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, 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
Basic EPS:
Net income	$ 1,070	$ 1,425	$ 1,305	$ 1,217	$ 1,239	$ 1,570	$ 1,405	$ 1,366	$ 5,017	$ 5,580	$ 3,682
Income allocated to RSUs									(32)	(43)	(34)
Income allocated to common stock for basic EPS									$ 4,985	$ 5,537	$ 3,648
Weighted average number of shares outstanding, basic (in shares)									936	970	991
Basic earnings per common share (in dollars per share)	$ 1.14	$ 1.51	$ 1.38	$ 1.29	$ 1.29	$ 1.61	$ 1.43	$ 1.38	$ 5.33	$ 5.71	$ 3.68
Dilutive effect of stock compensation plans									16	20	21
Diluted EPS:
Net income	$ 1,070	$ 1,425	$ 1,305	$ 1,217	$ 1,239	$ 1,570	$ 1,405	$ 1,366	$ 5,017	$ 5,580	$ 3,682
Income allocated to RSUs									(31)	(42)	(33)
Income allocated to common stock for diluted EPS									$ 4,986	$ 5,538	$ 3,649
Weighted average number of shares outstanding, diluted (in shares)									952	990	1,012
Diluted earnings per common share (in dollars per share)	$ 1.12	$ 1.49	$ 1.36	$ 1.26	$ 1.27	$ 1.58	$ 1.40	$ 1.35	$ 5.24	$ 5.59	$ 3.61