Loans and Allowance for Credit Losses - Summary of Loans Modified as TDRs (Detail) $ in Millions	3 Months Ended
Mar. 31, 2021 USD ($) SecurityLoan	Mar. 31, 2020 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	8,766	12,090
Pre-Modification Outstanding Loan Balance	$ 422	$ 457
Post-Modification Outstanding Loan Balance	$ 390	$ 453
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	704	999
Pre-Modification Outstanding Loan Balance	$ 75	$ 99
Post-Modification Outstanding Loan Balance	$ 60	$ 101
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	56	27
Pre-Modification Outstanding Loan Balance	$ 86	$ 21
Post-Modification Outstanding Loan Balance	$ 71	$ 21
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	336	90
Pre-Modification Outstanding Loan Balance	$ 104	$ 10
Post-Modification Outstanding Loan Balance	$ 104	$ 10
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	1,325	655
Pre-Modification Outstanding Loan Balance	$ 37	$ 15
Post-Modification Outstanding Loan Balance	$ 32	$ 14
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	5,786	8,415
Pre-Modification Outstanding Loan Balance	$ 33	$ 46
Post-Modification Outstanding Loan Balance	$ 34	$ 47
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	8,207	10,186
Pre-Modification Outstanding Loan Balance	$ 335	$ 191
Post-Modification Outstanding Loan Balance	$ 301	$ 193
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	559	1,904
Pre-Modification Outstanding Loan Balance	$ 87	$ 266
Post-Modification Outstanding Loan Balance	$ 89	$ 260

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Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953676-111524

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