Annual report pursuant to Section 13 and 15(d)

Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaps Associated with U.S. Treasuries and Other RMBS (Details)

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Derivative Instruments and Hedging Activities Schedule of Interest Rate Swaps Associated with U.S. Treasuries and Other RMBS (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Derivative [Line Items]    
Trading securities, at fair value $ 1,002,062 $ 1,003,301
Interest Rate Swaps Associated with U.S. Treasuries and Other RMBS [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 1,000,000 1,250,000
Interest Rate Swaps Associated with U.S. Treasuries and Other RMBS [Member] | Derivative Maturity Over Two And Within Three Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount   1,250,000
Derivative, Average Fixed Interest Rate   0.6196%
Derivative, Average Variable Interest Rate   0.3393%
Derivative, Weighted Average Remaining Maturity   1.535
Interest Rate Swaps Associated with U.S. Treasuries and Other RMBS [Member] | Derivative Maturity Over Three And Within Four Years From Balance Sheet Date [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount $ 1,000,000  
Derivative, Average Fixed Interest Rate 0.7991%  
Derivative, Average Variable Interest Rate 0.434%  
Derivative, Weighted Average Remaining Maturity 2.2778