Securities - Additional Information (Detail) (USD $)	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Investment Holdings [Line Items]
Trading securities	$ 177,000,000	$ 294,000,000
Securities with a fair value, pledged as collateral	13,300,000,000	11,300,000,000
Gross realized gains on trading securities	1,000,000	1,000,000	1,000,000
Gross realized losses on trading securities	7,000,000	1,000,000	2,000,000
Gross unrealized gains on trading securities	5,000,000
Gross unrealized losses on trading securities			8,000,000
Marketable Securities, Realized Loss, Other than Temporary Impairments, Amount	$ 19,000,000	$ 3,000,000
Available-for-sale Securities \| Noninvestment grade
Investment Holdings [Line Items]
Unrealized losses in the available-for-sale securities portfolio represented by non-rated securities		1.00%

The total of financial instruments that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for debt and equity securities formerly categorized as available-for-sale or held-to-maturity which the entity held as of the date it opted to account for such securities at fair value. An enterprise may also categorize such a security as trading without the intent to sell it in the near term assuming the decision to categorize the security as trading occurred at acquisition.

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-Name Accounting Standards Codification

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- Definition

The increase (decrease) resulting in a gain from the difference between the fair value and the carrying value, or in the comparative fair values, of trading securities held at each balance sheet date, that was included in earnings for the period.

+ References

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+ Details

- Definition

The increase (decrease) resulting in a loss from the difference between the fair value and the carrying value, or in the comparative fair values, of trading securities held at each balance sheet date, that was included in earnings for the period.

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