Loans and Related Allowance for Credit Losses (Credit Quality Indicators for Automobile, Credit Card, Education and Other Consumer Loan Classes) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 294,704	$ 241,928
Automobile | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	3,888	3,714
Financing receivable, originated in year one	4,360	5,494
Financing receivable, originated in year two	5,029	2,878
Financing receivable, originated in year three	2,639	1,361
Financing receivable, originated in year four	1,136	609
Prior	499	162
Total Loans	17,551	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	2,077	1,807
Financing receivable, originated in year one	1,816	1,915
Financing receivable, originated in year two	1,765	807
Financing receivable, originated in year three	730	452
Financing receivable, originated in year four	345	246
Prior	170	58
Total Loans	6,903	5,285
Automobile | 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	1,035	1,098
Financing receivable, originated in year one	1,304	1,581
Financing receivable, originated in year two	1,404	789
Financing receivable, originated in year three	697	381
Financing receivable, originated in year four	304	167
Prior	128	44
Total Loans	4,872	4,060
Automobile | 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	428	617
Financing receivable, originated in year one	827	1,222
Financing receivable, originated in year two	1,080	684
Financing receivable, originated in year three	606	288
Financing receivable, originated in year four	241	109
Prior	91	31
Total Loans	3,273	2,951
Automobile | Less than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	134	192
Financing receivable, originated in year one	380	776
Financing receivable, originated in year two	766	598
Financing receivable, originated in year three	599	240
Financing receivable, originated in year four	243	87
Prior	109	29
Total Loans	2,231	1,922
Automobile | No FICO score available or required | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	214
Financing receivable, originated in year one	33
Financing receivable, originated in year two	14
Financing receivable, originated in year three	7
Financing receivable, originated in year four	3
Prior	1
Total Loans	272
Credit card | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	6,449	6,120
Revolving Loans Converted to Term	79	95
Total Loans	6,528	6,215
Credit card | Greater than or equal to 780 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	1,754	1,635
Revolving Loans Converted to Term	3	3
Total Loans	1,757	1,638
Credit card | 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	1,805	1,724
Revolving Loans Converted to Term	10	11
Total Loans	1,815	1,735
Credit card | 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	1,841	1,765
Revolving Loans Converted to Term	25	26
Total Loans	1,866	1,791
Credit card | Less than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	912	902
Revolving Loans Converted to Term	37	51
Total Loans	949	953
Credit card | No FICO score available or required | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	137	94
Revolving Loans Converted to Term	4	4
Total Loans	141	98
Education
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	19	92
Financing receivable, originated in year one	118	163
Financing receivable, originated in year two	149	135
Financing receivable, originated in year three	121	101
Financing receivable, originated in year four	89	78
Prior	2,230	2,377
Total Loans	2,726	2,946
Education | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	19	92
Financing receivable, originated in year one	118	163
Financing receivable, originated in year two	149	135
Financing receivable, originated in year three	121	101
Financing receivable, originated in year four	89	78
Prior	731	746
Total Loans	1,227	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	9	34
Financing receivable, originated in year one	66	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	438	428
Total Loans	724	735
Education | 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	5	24
Financing receivable, originated in year one	31	46
Financing receivable, originated in year two	40	38
Financing receivable, originated in year three	32	28
Financing receivable, originated in year four	23	20
Prior	182	190
Total Loans	313	346
Education | 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	2	15
Financing receivable, originated in year one	9	15
Financing receivable, originated in year two	12	14
Financing receivable, originated in year three	12	9
Financing receivable, originated in year four	7	6
Prior	81	90
Total Loans	123	149
Education | Less than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year		3
Financing receivable, originated in year one	1	2
Financing receivable, originated in year two	2	3
Financing receivable, originated in year three	2	2
Financing receivable, originated in year four	2	2
Prior	29	37
Total Loans	36	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	11	10
Financing receivable, originated in year two	9	6
Financing receivable, originated in year three	5	3
Financing receivable, originated in year four	2
Prior	1	1
Total Loans	31	36
Education | Other internal credit metrics
Financing Receivable, Credit Quality Indicator [Line Items]
Prior	1,499	1,631
Total Loans	1,499	1,631
Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	581	581
Financing receivable, originated in year one	800	763
Financing receivable, originated in year two	846	304
Financing receivable, originated in year three	433	95
Financing receivable, originated in year four	127	75
Prior	216	161
Revolving Loans	3,204	2,713
Revolving Loans Converted to Term	32	6
Total Loans	6,239	4,698
Other consumer | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	527	514
Financing receivable, originated in year one	764	730
Financing receivable, originated in year two	819	267
Financing receivable, originated in year three	408	69
Financing receivable, originated in year four	101	15
Prior	104	86
Revolving Loans	708	379
Revolving Loans Converted to Term	12	3
Total Loans	3,443	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	133	162
Financing receivable, originated in year one	218	187
Financing receivable, originated in year two	201	63
Financing receivable, originated in year three	78	21
Financing receivable, originated in year four	26	5
Prior	45	42
Revolving Loans	235	86
Revolving Loans Converted to Term	7	1
Total Loans	943	567
Other consumer | 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	160	197
Financing receivable, originated in year one	258	247
Financing receivable, originated in year two	259	82
Financing receivable, originated in year three	110	22
Financing receivable, originated in year four	27	5
Prior	28	22
Revolving Loans	214	123
Revolving Loans Converted to Term	3
Total Loans	1,059	698
Other consumer | 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	120	127
Financing receivable, originated in year one	198	210
Financing receivable, originated in year two	243	81
Financing receivable, originated in year three	134	17
Financing receivable, originated in year four	29	3
Prior	18	14
Revolving Loans	162	117
Revolving Loans Converted to Term	1	1
Total Loans	905	570
Other consumer | Less than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	32	28
Financing receivable, originated in year one	81	86
Financing receivable, originated in year two	115	41
Financing receivable, originated in year three	85	9
Financing receivable, originated in year four	19	2
Prior	12	8
Revolving Loans	53	53
Revolving Loans Converted to Term	1	1
Total Loans	398	228
Other consumer | No FICO score available or required | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	82
Financing receivable, originated in year one	9
Financing receivable, originated in year two	1
Financing receivable, originated in year three	1
Prior	1
Revolving Loans	44
Total Loans	138
Other consumer | Other internal credit metrics
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in current year	54	67
Financing receivable, originated in year one	36	33
Financing receivable, originated in year two	27	37
Financing receivable, originated in year three	25	26
Financing receivable, originated in year four	26	60
Prior	112	75
Revolving Loans	2,496	2,334
Revolving Loans Converted to Term	20	3
Total Loans	$ 2,796	$ 2,635