Loans - Consumer Banking Portfolio by Credit Quality Indicator (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 243,131	$ 251,624	$ 262,990
Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	9,103	29,724
Term Loans by Vintage Year Two	27,169	18,253
Term Loans by Vintage Year Three	16,300	10,209
Term Loans by Vintage Year Four	8,881	6,191
Term Loans by Vintage Year Five	5,189	2,717
Term Loans by Vintage After Year Five	2,940	1,102
Total Term Loans	69,582	68,196
Revolving Loans	612	683
Revolving Loans Converted to Term	8	9
Total Loans	70,202	68,888
Auto \| Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	8,771	28,683
Term Loans by Vintage Year Two	26,289	18,020
Term Loans by Vintage Year Three	16,073	9,995
Term Loans by Vintage Year Four	8,674	5,967
Term Loans by Vintage Year Five	4,974	2,542
Term Loans by Vintage After Year Five	2,278	555
Total Term Loans	67,059	65,762
Revolving Loans	0	0
Revolving Loans Converted to Term	0	0
Total Loans	67,059	65,762
Retail banking \| Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	332	1,041
Term Loans by Vintage Year Two	880	233
Term Loans by Vintage Year Three	227	214
Term Loans by Vintage Year Four	207	224
Term Loans by Vintage Year Five	215	175
Term Loans by Vintage After Year Five	662	547
Total Term Loans	2,523	2,434
Revolving Loans	612	683
Revolving Loans Converted to Term	8	9
Total Loans	3,143	3,126
Loans under Paycheck Protection Program	1,100	919
Current \| Retail banking \| Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	332	1,041
Term Loans by Vintage Year Two	880	233
Term Loans by Vintage Year Three	227	206
Term Loans by Vintage Year Four	201	222
Term Loans by Vintage Year Five	212	167
Term Loans by Vintage After Year Five	650	537
Total Term Loans	2,502	2,406
Revolving Loans	582	651
Revolving Loans Converted to Term	7	7
Total Loans	3,091	3,064
30-59 days \| Retail banking \| Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	0	0
Term Loans by Vintage Year Two	0	0
Term Loans by Vintage Year Three	0	7
Term Loans by Vintage Year Four	5	1
Term Loans by Vintage Year Five	2	2
Term Loans by Vintage After Year Five	3	2
Total Term Loans	10	12
Revolving Loans	18	15
Revolving Loans Converted to Term	0	1
Total Loans	28	28
60-89 days \| Retail banking \| Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	0	0
Term Loans by Vintage Year Two	0	0
Term Loans by Vintage Year Three	0	1
Term Loans by Vintage Year Four	0	0
Term Loans by Vintage Year Five	0	5
Term Loans by Vintage After Year Five	5	4
Total Term Loans	5	10
Revolving Loans	3	8
Revolving Loans Converted to Term	0	1
Total Loans	8	19
Greater than 90 days \| Retail banking \| Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	0	0
Term Loans by Vintage Year Two	0	0
Term Loans by Vintage Year Three	0	0
Term Loans by Vintage Year Four	1	1
Term Loans by Vintage Year Five	1	1
Term Loans by Vintage After Year Five	4	4
Total Term Loans	6	6
Revolving Loans	9	9
Revolving Loans Converted to Term	1	0
Total Loans	16	15
Greater than 660 \| Auto \| Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	4,632	13,352
Term Loans by Vintage Year Two	12,126	8,091
Term Loans by Vintage Year Three	7,169	4,675
Term Loans by Vintage Year Four	4,010	2,810
Term Loans by Vintage Year Five	2,302	1,168
Term Loans by Vintage After Year Five	977	203
Total Term Loans	31,216	30,299
Revolving Loans	0	0
Revolving Loans Converted to Term	0	0
Total Loans	31,216	30,299
621-660 \| Auto \| Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	1,749	5,781
Term Loans by Vintage Year Two	5,311	3,631
Term Loans by Vintage Year Three	3,238	2,003
Term Loans by Vintage Year Four	1,749	1,172
Term Loans by Vintage Year Five	987	488
Term Loans by Vintage After Year Five	444	109
Total Term Loans	13,478	13,184
Revolving Loans	0	0
Revolving Loans Converted to Term	0	0
Total Loans	13,478	13,184
620 or below \| Auto \| Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year One	2,390	9,550
Term Loans by Vintage Year Two	8,852	6,298
Term Loans by Vintage Year Three	5,666	3,317
Term Loans by Vintage Year Four	2,915	1,985
Term Loans by Vintage Year Five	1,685	886
Term Loans by Vintage After Year Five	857	243
Total Term Loans	22,365	22,279
Revolving Loans	0	0
Revolving Loans Converted to Term	0	0
Total Loans	$ 22,365	$ 22,279

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