Quarterly Results (Schedule of Quarterly Results) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Data [Abstract]
Interest income	$ 2,760		$ 2,681		$ 2,672		$ 2,982		$ 3,039		$ 3,040		$ 2,977		$ 2,937		$ 11,095		$ 11,993		$ 10,893
Interest expense	383		416		482		584		615		638		645		632		1,865		2,530		2,139
Net interest income	2,377		2,265		2,190		2,398		2,424		2,402		2,332		2,305		9,230		9,463		8,754
Provision for credit losses	531		750		2,046		1,807		836		799		787		809		5,134	[1],[2]	3,231	[1],[2],[3]	3,035	[1],[2],[3]
Other income	447		449		472		490		520		498		520		458		1,858		1,996		1,955
Other expense	1,278		1,005		1,077		1,159		1,184		1,107		1,078		1,024		4,519		4,393		4,077
Income (loss) before income tax expense	1,015		959		(461)		(78)		924		994		987		930		1,435		3,835		3,597
Income tax expense	216		188		(93)		(17)		216		224		234		204		294		878		855
Net income	799		771		(368)		(61)		708		770		753		726		1,141		2,957		2,742
Net income allocated to common stockholders	$ 794	[4]	$ 751	[4]	$ (369)	[4]	$ (78)	[4]	$ 704	[4]	$ 749	[4]	$ 747	[4]	$ 705	[4]	$ 1,110		$ 2,926		$ 2,711
Basic earnings per common share	$ 2.59	[4]	$ 2.45	[4]	$ (1.20)	[4]	$ (0.25)	[4]	$ 2.25	[4]	$ 2.36	[4]	$ 2.32	[4]	$ 2.15	[4]	$ 3.60		$ 9.09		$ 7.81
Diluted earnings per common share	$ 2.59	[4]	$ 2.45	[4]	$ (1.20)	[4]	$ (0.25)	[4]	$ 2.25	[4]	$ 2.36	[4]	$ 2.32	[4]	$ 2.15	[4]	$ 3.60		$ 9.08		$ 7.79

[1] (1)Prior to adoption of ASU No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.

[2] Prior to adoption of ASU No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach.

[3] Prior to the adoption of ASU No. 2016-13 on January 1, 2020, credit losses were estimated using the incurred loss approach

[4] Because the inputs to net income allocated to common stockholders and earnings per share are calculated using weighted averages for the quarter, the sum of all four quarters may differ from the year to date amounts in the consolidated statements of income.