Securities - Additional Information (Detail) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment Holdings [Line Items]
Trading securities	$ 360,000,000us-gaap_TradingSecurities	$ 343,000,000us-gaap_TradingSecurities
Securities with a fair value, pledged as collateral	14,200,000,000	11,600,000,000
Gross realized gains on trading securities	4,000,000us-gaap_TradingSecuritiesRealizedGain	1,000,000us-gaap_TradingSecuritiesRealizedGain	2,000,000us-gaap_TradingSecuritiesRealizedGain
Net Unrealized Gain(Loss) on Trading Securities	(3,000,000)	3,000,000	1,000,000
Available-for-sale Securities
Investment Holdings [Line Items]
OTTI	$ (24,000,000)	$ (74,000,000)	$ (58,000,000)

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

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

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- Definition

This item represents the gain realized during the period from the sale of trading securities.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

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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.

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

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