Loans and Allowance for Credit Losses - Summary of Loans Modified as TDRs (Detail) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021 USD ($) SecurityLoan	Sep. 30, 2020 USD ($) SecurityLoan	Sep. 30, 2021 USD ($) SecurityLoan	Sep. 30, 2020 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	8,531	7,066	24,209	28,005
Pre-Modification Outstanding Loan Balance	$ 272	$ 634	$ 1,001	$ 1,486
Post-Modification Outstanding Loan Balance	$ 279	$ 506	$ 980	$ 1,321
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	506	699	1,736	2,837
Pre-Modification Outstanding Loan Balance	$ 46	$ 262	$ 133	$ 505
Post-Modification Outstanding Loan Balance	$ 47	$ 159	$ 120	$ 375
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	14	51	100	116
Pre-Modification Outstanding Loan Balance	$ 12	$ 105	$ 136	$ 165
Post-Modification Outstanding Loan Balance	$ 13	$ 81	$ 125	$ 141
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	171	374	867	585
Pre-Modification Outstanding Loan Balance	$ 54	$ 108	$ 299	$ 142
Post-Modification Outstanding Loan Balance	$ 54	$ 108	$ 298	$ 142
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	382	508	2,175	1,537
Pre-Modification Outstanding Loan Balance	$ 9	$ 26	$ 64	$ 50
Post-Modification Outstanding Loan Balance	$ 9	$ 26	$ 58	$ 48
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	6,656	4,699	17,492	19,282
Pre-Modification Outstanding Loan Balance	$ 38	$ 27	$ 102	$ 110
Post-Modification Outstanding Loan Balance	$ 38	$ 27	$ 103	$ 112
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	7,729	6,331	22,370	24,357
Pre-Modification Outstanding Loan Balance	$ 159	$ 528	$ 734	$ 972
Post-Modification Outstanding Loan Balance	$ 161	$ 401	$ 704	$ 818
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	802	735	1,839	3,648
Pre-Modification Outstanding Loan Balance	$ 113	$ 106	$ 267	$ 514
Post-Modification Outstanding Loan Balance	$ 118	$ 105	$ 276	$ 503

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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=123577603&loc=SL6953676-111524

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