Summary of Short-Term Borrowings and Weighted-Average Rates (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Federal funds purchased	$ 144us-gaap_FederalFundsPurchased	$ 284us-gaap_FederalFundsPurchased
Other short-term borrowings	1,556us-gaap_OtherShortTermBorrowings	1,380us-gaap_OtherShortTermBorrowings
Federal Funds Purchased
Short-term Debt [Line Items]
Federal funds purchased	144	284
Short-term borrowings, average	458	503
Short-term borrowings, maximum month-end balance	286	925
Short-term borrowngs, rate	0.08%	0.03%
Short-term borrowings, average rate	0.09%	0.12%
Other Short Term Borrowings
Short-term Debt [Line Items]
Other short-term borrowings	1,556	1,380
Short-term borrowings, average	1,873	3,024
Short-term borrowings, maximum month-end balance	$ 3,756	$ 8,001
Short-term borrowngs, rate	0.08%	0.09%
Short-term borrowings, average rate	0.10%	0.18%

Short Term Debt, During Period, End of Period Interest Rate

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- Definition

Amount of short term borrowing where a bank borrows, at the federal funds rate, from another bank.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.13(1))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 13

-Subparagraph 1

-Article 9

+ Details

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- Definition

Sum of the carrying amounts at the balance sheet date of short-term borrowings not otherwise specified in the taxonomy having initial terms less than one year or the normal operating cycle, if longer.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(a)(7))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, average borrowings during the period.

+ References

No definition available.

+ Details

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- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, maximum month-end borrowings during the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Industry Guide

-Number 3

-Section VII

-Subsection 2

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- Definition

Weighted average interest rate of short-term debt outstanding.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Subparagraph b

-Article 5

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