Investments in Affiliate, Available-For-Sale Equity Securities (Detail) (USD $) Share data in Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Chimera [Member]
Investments with Affiliate, Available-For-Sale Equity Securities:
Investments with affiliate, available-for-sale equity securities - shares	45.0	45.0
Investments with affiliate, available-for-sale equity securities - fair value	$ 117,400,000	$ 112,900,000
Investments with affiliate, available-for-sale equity securities - unrealized loss	21,500,000
CreXus [Member]
Investments with Affiliate, Available-For-Sale Equity Securities:
Investments with affiliate, available-for-sale equity securities - shares	9.5	9.5
Investments with affiliate, available-for-sale equity securities - fair value	116,700,000	98,900,000
Investments with affiliate, available-for-sale equity securities - unrealized loss	$ 8,700,000

Amount of equity securities categorized neither as held-to-maturity nor as trading.

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-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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

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

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

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-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.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-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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