Loans and Allowance for Credit Losses - Impaired Loans Average Recorded Investment and Interest Income Recognized (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	$ 4,920	$ 5,522
Interest Income Recognized	40	55
Commercial [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	545	817
Interest Income Recognized	1	1
Commercial Real Estate [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	267	278
Interest Income Recognized	2	2
Residential Mortgages [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	1,914	2,240
Interest Income Recognized	20	29
Other Retail [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	300	280
Interest Income Recognized	4	4
Credit Card [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	232	225
Interest Income Recognized	1	1
Total Loans, Excluding GNMA and Covered Loans [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	3,258	3,840
Interest Income Recognized	28	37
Covered Loans [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	38	36
Government National Mortgage Association [Member]
Financing Receivable, Impaired [Line Items]
Average Recorded Investment	1,624	1,646
Interest Income Recognized	$ 12	$ 18

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- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

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- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

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