Agency Mortgage-Backed Securities - Narrative (Detail) (USD $)	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Mortgage-Backed Securities Sold:
Mortgage-Backed securities sold, carrying value	$ 16,300,000,000	$ 5,100,000,000
Mortgage-Backed securities sold, realized gain	182,800,000	80,300,000
Interest-only securities
Mortgage-Backed Securities Sold:
Unrealized losses gains net	64,500,000
Unrealized losses	86,500,000
Unrealized gains	21,900,000
Amortized cost	$ 807,100,000

Carrying value of mortgage backed securities sold during period.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

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The net amount of gross unrealized gains (losses) for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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This item represents the gross unrealized losses for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

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No definition available.

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Net gain (loss) resulting from the sales of securitized, pay-through debt securities collateralized by real estate mortgage loans (mortgages).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

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