Loans and Allowance for Credit Losses - Summary of Loans Modified as TDRs (Detail) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	12,788	10,793
Pre-Modification Outstanding Loan Balance	$ 361	$ 285
Post-Modification Outstanding Loan Balance	$ 344	$ 273
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	913	623
Pre-Modification Outstanding Loan Balance	$ 36	$ 81
Post-Modification Outstanding Loan Balance	$ 29	$ 75
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	20	29
Pre-Modification Outstanding Loan Balance	$ 47	$ 16
Post-Modification Outstanding Loan Balance	$ 46	$ 16
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	96	148
Pre-Modification Outstanding Loan Balance	$ 14	$ 17
Post-Modification Outstanding Loan Balance	$ 13	$ 16
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	573	559
Pre-Modification Outstanding Loan Balance	$ 11	$ 11
Post-Modification Outstanding Loan Balance	$ 10	$ 10
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	9,648	8,546
Pre-Modification Outstanding Loan Balance	$ 50	$ 43
Post-Modification Outstanding Loan Balance	$ 51	$ 43
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	11,250	9,905
Pre-Modification Outstanding Loan Balance	$ 158	$ 168
Post-Modification Outstanding Loan Balance	$ 149	$ 160
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans \| SecurityLoan	1,538	888
Pre-Modification Outstanding Loan Balance	$ 203	$ 117
Post-Modification Outstanding Loan Balance	$ 195	$ 113

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+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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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