Securities - Additional Information (Detail) (USD $)	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Investment Holdings [Line Items]
Trading securities	$ 392,000,000us-gaap_TradingSecurities		$ 360,000,000us-gaap_TradingSecurities
Securities with a fair value, pledged as collateral	14,100,000,000		14,200,000,000
Net Unrealized Gain(Loss) on Trading Securities	1,000,000	1,000,000
Available-for-sale Securities
Investment Holdings [Line Items]
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 1,000,000	$ 17,000,000

The fair value of available-for-sale securities that serve as collateral for borrowings.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 5

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- Definition

This element represents the amount of loss recognized for other than temporary impairments (OTTI) of investments in debt and equity securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

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

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- Definition

Amount of unrealized holding gain (loss) recognized in the income statement for investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

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

-Publisher FASB

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