Loans and Related Allowance for Credit Losses (Credit Quality Indicators for Automobile, Credit Card, Education and Other Consumer Loan Classes) (Details) $ in Millions	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 249,279	$ 239,843
Automobile Loan [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		16,754
Automobile Loan [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	2,979
Financing receivable, originated in 2019	6,112
Financing receivable, originated in 2018	3,279
Financing receivable, originated in 2017	1,604
Financing receivable, originated in 2016	767
Prior	236
Total Loans	14,977	$ 16,754
Weighted average updated FICO score		726
Automobile Loan [Member] | FICO Score - Greater than 719 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	2,184
Financing receivable, originated in 2019	3,573
Financing receivable, originated in 2018	1,663
Financing receivable, originated in 2017	916
Financing receivable, originated in 2016	496
Prior	141
Total Loans	8,973	$ 9,232
Automobile Loan [Member] | FICO Score - 650 to 719 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	630
Financing receivable, originated in 2019	1,642
Financing receivable, originated in 2018	929
Financing receivable, originated in 2017	403
Financing receivable, originated in 2016	162
Prior	53
Total Loans	3,819	4,577
Automobile Loan [Member] | FICO Score - 620 to 649 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	90
Financing receivable, originated in 2019	365
Financing receivable, originated in 2018	213
Financing receivable, originated in 2017	83
Financing receivable, originated in 2016	31
Prior	12
Total Loans	794	1,001
Automobile Loan [Member] | FICO Score - Less than 620 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	75
Financing receivable, originated in 2019	532
Financing receivable, originated in 2018	474
Financing receivable, originated in 2017	202
Financing receivable, originated in 2016	78
Prior	30
Total Loans	1,391	1,603
Automobile Loan [Member] | No FICO Score Available Or Required [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		341
Credit Card [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		7,308
Credit Card [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	6,203
Revolving Loans Converted to Term	100
Total Loans	6,303	$ 7,308
Weighted average updated FICO score		724
Credit Card [Member] | FICO Score - Greater than 719 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	3,309
Revolving Loans Converted to Term	13
Total Loans	3,322	$ 3,867
Credit Card [Member] | FICO Score - 650 to 719 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	2,033
Revolving Loans Converted to Term	31
Total Loans	2,064	2,326
Credit Card [Member] | FICO Score - 620 to 649 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	348
Revolving Loans Converted to Term	13
Total Loans	361	419
Credit Card [Member] | FICO Score - Less than 620 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	419
Revolving Loans Converted to Term	40
Total Loans	459	544
Credit Card [Member] | No FICO Score Available Or Required [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Revolving Loans	94
Revolving Loans Converted to Term	3
Total Loans	97	152
Education [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	64
Financing receivable, originated in 2019	167
Financing receivable, originated in 2018	141
Financing receivable, originated in 2017	105
Financing receivable, originated in 2016	83
Prior	2,491
Total Loans	3,051	3,336
Education [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	34
Financing receivable, originated in 2019	109
Financing receivable, originated in 2018	141
Financing receivable, originated in 2017	105
Financing receivable, originated in 2016	83
Prior	785
Total Loans	1,257	$ 1,403
Weighted average updated FICO score		773
Education [Member] | FICO Score - Greater than 719 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	17
Financing receivable, originated in 2019	88
Financing receivable, originated in 2018	117
Financing receivable, originated in 2017	89
Financing receivable, originated in 2016	73
Prior	651
Total Loans	1,035	$ 1,139
Education [Member] | FICO Score - 650 to 719 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	6
Financing receivable, originated in 2019	10
Financing receivable, originated in 2018	14
Financing receivable, originated in 2017	9
Financing receivable, originated in 2016	7
Prior	102
Total Loans	148	197
Education [Member] | FICO Score - 620 to 649 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2019	1
Financing receivable, originated in 2018	2
Financing receivable, originated in 2017	1
Financing receivable, originated in 2016	1
Prior	15
Total Loans	20	25
Education [Member] | FICO Score - Less than 620 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2018	1
Financing receivable, originated in 2017	1
Financing receivable, originated in 2016	1
Prior	16
Total Loans	19	27
Education [Member] | No FICO Score Available Or Required [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	11
Financing receivable, originated in 2019	10
Financing receivable, originated in 2018	7
Financing receivable, originated in 2017	5
Financing receivable, originated in 2016	1
Prior	1
Total Loans	35	15
Education [Member] | Other Internal Credit Metrics [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	30
Financing receivable, originated in 2019	58
Prior	1,706
Total Loans	1,794	1,933
Other Consumer [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	551
Financing receivable, originated in 2019	881
Financing receivable, originated in 2018	359
Financing receivable, originated in 2017	114
Financing receivable, originated in 2016	84
Prior	176
Revolving Loans	2,647
Revolving Loans Converted to Term	17
Total Loans	4,829	4,937
Other Consumer [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	488
Financing receivable, originated in 2019	840
Financing receivable, originated in 2018	319
Financing receivable, originated in 2017	86
Financing receivable, originated in 2016	23
Prior	99
Revolving Loans	401
Revolving Loans Converted to Term	3
Total Loans	2,259	$ 2,566
Weighted average updated FICO score		727
Other Consumer [Member] | FICO Score - Greater than 719 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	338
Financing receivable, originated in 2019	487
Financing receivable, originated in 2018	164
Financing receivable, originated in 2017	50
Financing receivable, originated in 2016	14
Prior	69
Revolving Loans	209
Revolving Loans Converted to Term	1
Total Loans	1,332	$ 1,421
Other Consumer [Member] | FICO Score - 650 to 719 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	129
Financing receivable, originated in 2019	273
Financing receivable, originated in 2018	112
Financing receivable, originated in 2017	25
Financing receivable, originated in 2016	6
Prior	19
Revolving Loans	138
Revolving Loans Converted to Term	1
Total Loans	703	843
Other Consumer [Member] | FICO Score - 620 to 649 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	12
Financing receivable, originated in 2019	42
Financing receivable, originated in 2018	18
Financing receivable, originated in 2017	4
Financing receivable, originated in 2016	1
Prior	4
Revolving Loans	22
Total Loans	103	132
Other Consumer [Member] | FICO Score - Less than 620 [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	9
Financing receivable, originated in 2019	38
Financing receivable, originated in 2018	25
Financing receivable, originated in 2017	7
Financing receivable, originated in 2016	2
Prior	7
Revolving Loans	32
Revolving Loans Converted to Term	1
Total Loans	121	143
Other Consumer [Member] | No FICO Score Available Or Required [Member] | Using FICO Credit Metric [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans		27
Other Consumer [Member] | Other Internal Credit Metrics [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, originated in 2020	63
Financing receivable, originated in 2019	41
Financing receivable, originated in 2018	40
Financing receivable, originated in 2017	28
Financing receivable, originated in 2016	61
Prior	77
Revolving Loans	2,246
Revolving Loans Converted to Term	14
Total Loans	$ 2,570	$ 2,371