Securities - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Investment Holdings [Line Items]
Trading securities	$ 195,000,000	$ 216,000,000	$ 177,000,000
Securities with a fair value, pledged as collateral	12,600,000,000	10,600,000,000	13,300,000,000
Net Unrealized Gain(Loss) on Trading Securities	$ 2,000,000	$ (1,000,000)	$ 5,000,000
Available-for-sale Securities \| Noninvestment grade
Investment Holdings [Line Items]
Unrealized losses in the available-for-sale securities portfolio represented by non-rated securities		2.00%

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.

Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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

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+ Details

- Definition

The net change in 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. Trading securities are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) and the change in net unrealized holding gain (loss) is included in earnings for the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

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+ Details