Loans and Allowance for Credit Losses - Summary of Loans Modified as TDRs (Detail) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	10,793	14,169
Pre-Modification Outstanding Loan Balance	$ 285	$ 630
Post-Modification Outstanding Loan Balance	$ 273	$ 611
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	623	830
Pre-Modification Outstanding Loan Balance	$ 81	$ 137
Post-Modification Outstanding Loan Balance	$ 75	$ 128
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	29	23
Pre-Modification Outstanding Loan Balance	$ 16	$ 9
Post-Modification Outstanding Loan Balance	$ 16	$ 8
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	148	356
Pre-Modification Outstanding Loan Balance	$ 17	$ 40
Post-Modification Outstanding Loan Balance	$ 16	$ 41
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	559	622
Pre-Modification Outstanding Loan Balance	$ 11	$ 11
Post-Modification Outstanding Loan Balance	$ 10	$ 9
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	8,546	9,405
Pre-Modification Outstanding Loan Balance	$ 43	$ 45
Post-Modification Outstanding Loan Balance	$ 43	$ 46
Total Loans, Excluding GNMA and Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	9,905	11,236
Pre-Modification Outstanding Loan Balance	$ 168	$ 242
Post-Modification Outstanding Loan Balance	$ 160	$ 232
Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan		4
Pre-Modification Outstanding Loan Balance		$ 1
Post-Modification Outstanding Loan Balance		$ 1
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	888	2,929
Pre-Modification Outstanding Loan Balance	$ 117	$ 387
Post-Modification Outstanding Loan Balance	$ 113	$ 378

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

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