Asset Quality - Nonperforming Assets and Past Due Loans (Parenthetical) (Detail) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Financing Receivable, Recorded Investment, Past Due [Line Items]
Purchased credit impaired loans	$ 12	$ 13	$ 14
Restructured loans	20	$ 17	$ 16
Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Carrying value of foreclosed residential real estate	114
Foreclosed Residential Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Carrying value of foreclosed residential real estate	$ 13

Purchased credit impaired loans outstanding principal balance.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

The amount of mortgage loans on real estate which were foreclosed on during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 948

-SubTopic 310

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-29.6)

-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule IV)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 29

-Article 12

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule IV

-Article 5

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- Definition

Amount of restructured debt contingently payable based on the occurrence of future events or circumstances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 60

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6402221&loc=d3e15765-112638

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