Loans - Consumer Banking Portfolio by Credit Quality Indicator (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
Total	$ 315,154	$ 320,472
Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	7,481	23,828
Term Loans by Vintage Year, Year 2	21,972	22,721
Term Loans by Vintage Year, Year 3	20,591	17,192
Term Loans by Vintage Year, Year 4	15,295	6,876
Term Loans by Vintage Year, Year 5	5,889	3,053
Prior	3,495	1,380
Total Term Loans	74,723	75,050
Revolving Loans	370	382
Revolving Loans Converted to Term	6	5
Total	75,099	75,437
Auto | Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	7,412	23,729
Term Loans by Vintage Year, Year 2	21,895	22,564
Term Loans by Vintage Year, Year 3	20,499	17,134
Term Loans by Vintage Year, Year 4	15,238	6,810
Term Loans by Vintage Year, Year 5	5,826	2,936
Prior	2,931	902
Total Term Loans	73,801	74,075
Revolving Loans	0	0
Revolving Loans Converted to Term	0	0
Total	73,801	74,075
Retail banking | Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	69	99
Term Loans by Vintage Year, Year 2	77	157
Term Loans by Vintage Year, Year 3	92	58
Term Loans by Vintage Year, Year 4	57	66
Term Loans by Vintage Year, Year 5	63	117
Prior	564	478
Total Term Loans	922	975
Revolving Loans	370	382
Revolving Loans Converted to Term	6	5
Total	1,298	1,362
Current | Retail banking | Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	69	98
Term Loans by Vintage Year, Year 2	77	157
Term Loans by Vintage Year, Year 3	92	57
Term Loans by Vintage Year, Year 4	56	65
Term Loans by Vintage Year, Year 5	62	117
Prior	554	468
Total Term Loans	910	962
Revolving Loans	355	363
Revolving Loans Converted to Term	4	4
Total	1,269	1,329
30-59 days | Retail banking | Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	0	1
Term Loans by Vintage Year, Year 2	0	0
Term Loans by Vintage Year, Year 3	0	1
Term Loans by Vintage Year, Year 4	1	1
Term Loans by Vintage Year, Year 5	0	0
Prior	2	1
Total Term Loans	3	4
Revolving Loans	8	11
Revolving Loans Converted to Term	0	0
Total	11	15
60-89 days | Retail banking | Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	0	0
Term Loans by Vintage Year, Year 2	0	0
Term Loans by Vintage Year, Year 3	0	0
Term Loans by Vintage Year, Year 4	0	0
Term Loans by Vintage Year, Year 5	0	0
Prior	1	1
Total Term Loans	1	1
Revolving Loans	2	2
Revolving Loans Converted to Term	0	0
Total	3	3
Greater than 90 days | Retail banking | Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	0	0
Term Loans by Vintage Year, Year 2	0	0
Term Loans by Vintage Year, Year 3	0	0
Term Loans by Vintage Year, Year 4	0	0
Term Loans by Vintage Year, Year 5	1	0
Prior	7	8
Total Term Loans	8	8
Revolving Loans	5	6
Revolving Loans Converted to Term	2	1
Total	15	15
Greater than 660 | Auto | Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	4,180	12,219
Term Loans by Vintage Year, Year 2	11,141	12,593
Term Loans by Vintage Year, Year 3	11,433	9,505
Term Loans by Vintage Year, Year 4	8,457	3,124
Term Loans by Vintage Year, Year 5	2,654	1,213
Prior	1,132	309
Total Term Loans	38,997	38,963
Revolving Loans	0	0
Revolving Loans Converted to Term	0	0
Total	38,997	38,963
621-660 | Auto | Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	1,367	4,863
Term Loans by Vintage Year, Year 2	4,526	4,432
Term Loans by Vintage Year, Year 3	4,030	3,346
Term Loans by Vintage Year, Year 4	2,974	1,337
Term Loans by Vintage Year, Year 5	1,148	592
Prior	600	192
Total Term Loans	14,645	14,762
Revolving Loans	0	0
Revolving Loans Converted to Term	0	0
Total	14,645	14,762
620 or below | Auto | Consumer Banking:
Financing Receivable, Credit Quality Indicator [Line Items]
Term Loans by Vintage Year, Year 1	1,865	6,647
Term Loans by Vintage Year, Year 2	6,228	5,539
Term Loans by Vintage Year, Year 3	5,036	4,283
Term Loans by Vintage Year, Year 4	3,807	2,349
Term Loans by Vintage Year, Year 5	2,024	1,131
Prior	1,199	401
Total Term Loans	20,159	20,350
Revolving Loans	0	0
Revolving Loans Converted to Term	0	0
Total	$ 20,159	$ 20,350