Loans and Related Allowance for Credit Losses (Credit Quality Indicators for Automobile, Credit Card, Education and Other Consumer Loan Classes) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 237,013	$ 241,928
Automobile | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	1,084	3,714
Financing receivable, originated in year one	3,431	5,494
Financing receivable, originated in year two	4,880	2,878
Financing receivable, originated in year three	2,491	1,361
Financing receivable, originated in year four	1,130	609
Prior	568	162
Total Loans	13,584	14,218
Automobile | Greater than or equal to 780 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	775	1,807
Financing receivable, originated in year one	1,545	1,915
Financing receivable, originated in year two	1,701	807
Financing receivable, originated in year three	688	452
Financing receivable, originated in year four	362	246
Prior	216	58
Total Loans	5,287	5,285
Automobile | 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	242	1,098
Financing receivable, originated in year one	1,067	1,581
Financing receivable, originated in year two	1,380	789
Financing receivable, originated in year three	678	381
Financing receivable, originated in year four	320	167
Prior	157	44
Total Loans	3,844	4,060
Automobile | 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	63	617
Financing receivable, originated in year one	591	1,222
Financing receivable, originated in year two	1,076	684
Financing receivable, originated in year three	590	288
Financing receivable, originated in year four	240	109
Prior	106	31
Total Loans	2,666	2,951
Automobile | Less than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	4	192
Financing receivable, originated in year one	228	776
Financing receivable, originated in year two	723	598
Financing receivable, originated in year three	535	240
Financing receivable, originated in year four	208	87
Prior	89	29
Total Loans	1,787	1,922
Credit card | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	5,586	6,120
Revolving Loans Converted to Term	89	95
Total Loans	5,675	6,215
Credit card | Greater than or equal to 780 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	1,513	1,635
Revolving Loans Converted to Term	3	3
Total Loans	1,516	1,638
Credit card | 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	1,541	1,724
Revolving Loans Converted to Term	11	11
Total Loans	1,552	1,735
Credit card | 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	1,621	1,765
Revolving Loans Converted to Term	25	26
Total Loans	1,646	1,791
Credit card | Less than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	817	902
Revolving Loans Converted to Term	46	51
Total Loans	863	953
Credit card | No FICO Score Available Or Required | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	94	94
Revolving Loans Converted to Term	4	4
Total Loans	98	98
Education
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	13	92
Financing receivable, originated in year one	121	163
Financing receivable, originated in year two	156	135
Financing receivable, originated in year three	128	101
Financing receivable, originated in year four	95	78
Prior	2,329	2,377
Total Loans	2,842	2,946
Education | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	13	92
Financing receivable, originated in year one	121	163
Financing receivable, originated in year two	156	135
Financing receivable, originated in year three	128	101
Financing receivable, originated in year four	95	78
Prior	778	746
Total Loans	1,291	1,315
Education | Greater than or equal to 780 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	1	34
Financing receivable, originated in year one	64	90
Financing receivable, originated in year two	86	74
Financing receivable, originated in year three	70	59
Financing receivable, originated in year four	55	50
Prior	450	428
Total Loans	726	735
Education | 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	4	24
Financing receivable, originated in year one	34	46
Financing receivable, originated in year two	44	38
Financing receivable, originated in year three	37	28
Financing receivable, originated in year four	27	20
Prior	202	190
Total Loans	348	346
Education | 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	4	15
Financing receivable, originated in year one	10	15
Financing receivable, originated in year two	14	14
Financing receivable, originated in year three	14	9
Financing receivable, originated in year four	8	6
Prior	90	90
Total Loans	140	149
Education | Less than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	1	3
Financing receivable, originated in year one	1	2
Financing receivable, originated in year two	3	3
Financing receivable, originated in year three	2	2
Financing receivable, originated in year four	2	2
Prior	35	37
Total Loans	44	49
Education | No FICO Score Available Or Required | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	3	16
Financing receivable, originated in year one	12	10
Financing receivable, originated in year two	9	6
Financing receivable, originated in year three	5	3
Financing receivable, originated in year four	3
Prior	1	1
Total Loans	33	36
Education | Other internal credit metrics
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year
Financing receivable, originated in year one
Prior	1,551	1,631
Total Loans	1,551	1,631
Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	124	581
Financing receivable, originated in year one	478	763
Financing receivable, originated in year two	650	304
Financing receivable, originated in year three	245	95
Financing receivable, originated in year four	77	75
Prior	210	161
Revolving Loans	2,701	2,713
Revolving Loans Converted to Term	10	6
Total Loans	4,495	4,698
Other consumer | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	92	514
Financing receivable, originated in year one	449	730
Financing receivable, originated in year two	622	267
Financing receivable, originated in year three	216	69
Financing receivable, originated in year four	51	15
Prior	84	86
Revolving Loans	336	379
Revolving Loans Converted to Term	2	3
Total Loans	1,852	2,063
Other consumer | Greater than or equal to 780 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	31	162
Financing receivable, originated in year one	135	187
Financing receivable, originated in year two	150	63
Financing receivable, originated in year three	49	21
Financing receivable, originated in year four	15	5
Prior	40	42
Revolving Loans	73	86
Revolving Loans Converted to Term	1	1
Total Loans	494	567
Other consumer | 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	33	197
Financing receivable, originated in year one	169	247
Financing receivable, originated in year two	206	82
Financing receivable, originated in year three	65	22
Financing receivable, originated in year four	16	5
Prior	23	22
Revolving Loans	109	123
Revolving Loans Converted to Term
Total Loans	621	698
Other consumer | 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	27	127
Financing receivable, originated in year one	111	210
Financing receivable, originated in year two	185	81
Financing receivable, originated in year three	68	17
Financing receivable, originated in year four	13	3
Prior	14	14
Revolving Loans	107	117
Revolving Loans Converted to Term		1
Total Loans	525	570
Other consumer | Less than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	1	28
Financing receivable, originated in year one	34	86
Financing receivable, originated in year two	81	41
Financing receivable, originated in year three	34	9
Financing receivable, originated in year four	7	2
Prior	7	8
Revolving Loans	47	53
Revolving Loans Converted to Term	1	1
Total Loans	212	228
Other consumer | Other internal credit metrics
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	32	67
Financing receivable, originated in year one	29	33
Financing receivable, originated in year two	28	37
Financing receivable, originated in year three	29	26
Financing receivable, originated in year four	26	60
Prior	126	75
Revolving Loans	2,365	2,334
Revolving Loans Converted to Term	8	3
Total Loans	$ 2,643	$ 2,635