Loans Loans - Consumer Banking Portfolio by Credit Quality Indicator (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Total Loans	$ 248,223	$ 265,809	$ 249,355
Loans under Paycheck Protection Program	242
Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	24,184
2019	20,209
2018	11,570
2017	7,216
2016	3,350
Prior	1,422
Total Term Loans	67,951
Revolving Loans	727
Revolving Loans Converted to Term	10
Total Loans	68,688	63,065
Total Loans, Excluding PCI Loans		63,063
Auto | Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	23,103
2019	19,972
2018	11,339
2017	6,982
2016	3,161
Prior	837
Total Term Loans	65,394
Revolving Loans	0
Revolving Loans Converted to Term	0
Total Loans	65,394	60,362
Total Loans, Excluding PCI Loans		60,362
Retail banking | Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,081
2019	237
2018	231
2017	234
2016	189
Prior	585
Total Term Loans	2,557
Revolving Loans	727
Revolving Loans Converted to Term	10
Total Loans	3,294	2,703
Total Loans, Excluding PCI Loans		2,701
Loans under Paycheck Protection Program	966
Current | Retail banking | Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,081
2019	237
2018	231
2017	231
2016	186
Prior	576
Total Term Loans	2,542
Revolving Loans	699
Revolving Loans Converted to Term	9
Total Loans	3,250
Total Loans, Excluding PCI Loans		2,658
30-59 days | Retail banking | Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	2
2016	1
Prior	3
Total Term Loans	6
Revolving Loans	17
Revolving Loans Converted to Term	0
Total Loans	23
Total Loans, Excluding PCI Loans		24
60-89 days | Retail banking | Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	1
Prior	2
Total Term Loans	3
Revolving Loans	3
Revolving Loans Converted to Term	0
Total Loans	6
Total Loans, Excluding PCI Loans		8
Greater than 90 days | Retail banking | Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	1
2016	1
Prior	4
Total Term Loans	6
Revolving Loans	8
Revolving Loans Converted to Term	1
Total Loans	15
Total Loans, Excluding PCI Loans		11
Greater than 660 | Auto | Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	10,562
2019	9,021
2018	5,353
2017	3,338
2016	1,496
Prior	333
Total Term Loans	30,103
Revolving Loans	0
Revolving Loans Converted to Term	0
Total Loans	30,103
Total Loans, Excluding PCI Loans		28,773
621-660 | Auto | Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	4,666
2019	4,027
2018	2,264
2017	1,361
2016	597
Prior	161
Total Term Loans	13,076
Revolving Loans	0
Revolving Loans Converted to Term	0
Total Loans	13,076
Total Loans, Excluding PCI Loans		11,924
620 or below | Auto | Consumer Banking
Financing Receivable, Credit Quality Indicator [Line Items]
2020	7,875
2019	6,924
2018	3,722
2017	2,283
2016	1,068
Prior	343
Total Term Loans	22,215
Revolving Loans	0
Revolving Loans Converted to Term	0
Total Loans	$ 22,215
Total Loans, Excluding PCI Loans		$ 19,665