Loans and Related Allowance for Credit Losses (Credit Quality Indicators for Automobile, Credit Card, Education and Other Consumer Loan Classes) (Details) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 241,928	$ 239,843	$ 226,245
Automobile
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		16,754
Automobile | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3,714
2019	5,494
2018	2,878
2017	1,361
2016	609
Prior	162
Total Loans	14,218	$ 16,754
Weighted average FICO score		726
Automobile | FICO Score, Greater Than Or Equal To 780 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,807
2019	1,915
2018	807
2017	452
2016	246
Prior	58
Total Loans	5,285
Automobile | FICO Score, 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,098
2019	1,581
2018	789
2017	381
2016	167
Prior	44
Total Loans	4,060
Automobile | FICO Score, 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	617
2019	1,222
2018	684
2017	288
2016	109
Prior	31
Total Loans	2,951
Automobile | FICO Score, Less Than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	192
2019	776
2018	598
2017	240
2016	87
Prior	29
Total Loans	1,922
Automobile | No FICO Score Available Or Required | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		$ 341
Automobile | FICO Score - Greater than 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		9,232
Automobile | FICO Score - 650 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		4,577
Automobile | FICO Score - 620 to 649 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		1,001
Automobile | FICO Score - Less than 620 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		1,603
Credit card
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		7,308
Credit card | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	6,120
Revolving Loans Converted to Term	95
Total Loans	6,215	$ 7,308
Weighted average FICO score		724
Credit card | FICO Score, Greater Than Or Equal To 780 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	1,635
Revolving Loans Converted to Term	3
Total Loans	1,638
Credit card | FICO Score, 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	1,724
Revolving Loans Converted to Term	11
Total Loans	1,735
Credit card | FICO Score, 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	1,765
Revolving Loans Converted to Term	26
Total Loans	1,791
Credit card | FICO Score, Less Than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	902
Revolving Loans Converted to Term	51
Total Loans	953
Credit card | No FICO Score Available Or Required | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	94
Revolving Loans Converted to Term	4
Total Loans	98	$ 152
Credit card | FICO Score - Greater than 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		3,867
Credit card | FICO Score - 650 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		2,326
Credit card | FICO Score - 620 to 649 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		419
Credit card | FICO Score - Less than 620 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		544
Education
Financing Receivable, Credit Quality Indicator [Line Items]
2020	92
2019	163
2018	135
2017	101
2016	78
Prior	2,377
Total Loans	2,946	3,336
Education | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	92
2019	163
2018	135
2017	101
2016	78
Prior	746
Total Loans	1,315	$ 1,403
Weighted average FICO score		773
Education | FICO Score, Greater Than Or Equal To 780 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	34
2019	90
2018	74
2017	59
2016	50
Prior	428
Total Loans	735
Education | FICO Score, 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	24
2019	46
2018	38
2017	28
2016	20
Prior	190
Total Loans	346
Education | FICO Score, 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	15
2019	15
2018	14
2017	9
2016	6
Prior	90
Total Loans	149
Education | FICO Score, Less Than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	3
2019	2
2018	3
2017	2
2016	2
Prior	37
Total Loans	49
Education | No FICO Score Available Or Required | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	16
2019	10
2018	6
2017	3
2016
Prior	1
Total Loans	36	$ 15
Education | Other Internal Credit Metrics
Financing Receivable, Credit Quality Indicator [Line Items]
Prior	1,631
Total Loans	1,631	1,933
Education | FICO Score - Greater than 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		1,139
Education | FICO Score - 650 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		197
Education | FICO Score - 620 to 649 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		25
Education | FICO Score - Less than 620 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		27
Other consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2020	581
2019	763
2018	304
2017	95
2016	75
Prior	161
Revolving Loans	2,713
Revolving Loans Converted to Term	6
Total Loans	4,698	4,937
Other consumer | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	514
2019	730
2018	267
2017	69
2016	15
Prior	86
Revolving Loans	379
Revolving Loans Converted to Term	3
Total Loans	2,063	$ 2,566
Weighted average FICO score		727
Other consumer | FICO Score, Greater Than Or Equal To 780 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	162
2019	187
2018	63
2017	21
2016	5
Prior	42
Revolving Loans	86
Revolving Loans Converted to Term	1
Total Loans	567
Other consumer | FICO Score, 720 to 779 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	197
2019	247
2018	82
2017	22
2016	5
Prior	22
Revolving Loans	123
Total Loans	698
Other consumer | FICO Score, 660 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	127
2019	210
2018	81
2017	17
2016	3
Prior	14
Revolving Loans	117
Revolving Loans Converted to Term	1
Total Loans	570
Other consumer | FICO Score, Less Than 660 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
2020	28
2019	86
2018	41
2017	9
2016	2
Prior	8
Revolving Loans	53
Revolving Loans Converted to Term	1
Total Loans	228
Other consumer | No FICO Score Available Or Required | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		$ 27
Other consumer | Other Internal Credit Metrics
Financing Receivable, Credit Quality Indicator [Line Items]
2020	67
2019	33
2018	37
2017	26
2016	60
Prior	75
Revolving Loans	2,334
Revolving Loans Converted to Term	3
Total Loans	$ 2,635	2,371
Other consumer | FICO Score - Greater than 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		1,421
Other consumer | FICO Score - 650 to 719 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		843
Other consumer | FICO Score - 620 to 649 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		132
Other consumer | FICO Score - Less than 620 | Using FICO Credit Metric
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		$ 143