Loans and Allowance for Credit Losses - Summary of Loans Modified as TDRs (Detail) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021 USD ($) SecurityLoan	Jun. 30, 2020 USD ($) SecurityLoan	Jun. 30, 2021 USD ($) SecurityLoan	Jun. 30, 2020 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	6,912	8,849	15,678	20,939
Pre-Modification Outstanding Loan Balance	$ 307	$ 395	$ 729	$ 852
Post-Modification Outstanding Loan Balance	$ 311	$ 362	$ 701	$ 815
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	526	1,139	1,230	2,138
Pre-Modification Outstanding Loan Balance	$ 12	$ 144	$ 87	$ 243
Post-Modification Outstanding Loan Balance	$ 13	$ 115	$ 73	$ 216
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	30	38	86	65
Pre-Modification Outstanding Loan Balance	$ 38	$ 39	$ 124	$ 60
Post-Modification Outstanding Loan Balance	$ 41	$ 39	$ 112	$ 60
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	360	121	696	211
Pre-Modification Outstanding Loan Balance	$ 141	$ 24	$ 245	$ 34
Post-Modification Outstanding Loan Balance	$ 140	$ 24	$ 244	$ 34
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	468	374	1,793	1,029
Pre-Modification Outstanding Loan Balance	$ 18	$ 9	$ 55	$ 24
Post-Modification Outstanding Loan Balance	$ 17	$ 8	$ 49	$ 22
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	5,050	6,168	10,836	14,583
Pre-Modification Outstanding Loan Balance	$ 31	$ 37	$ 64	$ 83
Post-Modification Outstanding Loan Balance	$ 31	$ 38	$ 65	$ 85
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	6,434	7,840	14,641	18,026
Pre-Modification Outstanding Loan Balance	$ 240	$ 253	$ 575	$ 444
Post-Modification Outstanding Loan Balance	$ 242	$ 224	$ 543	$ 417
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	478	1,009	1,037	2,913
Pre-Modification Outstanding Loan Balance	$ 67	$ 142	$ 154	$ 408
Post-Modification Outstanding Loan Balance	$ 69	$ 138	$ 158	$ 398

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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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