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Derivative Instruments and Hedging Activities Schedule of Derivative Instruments, Gain (Loss) in Statement of Financial Performance (Details)

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Derivative Instruments and Hedging Activities Schedule of Derivative Instruments, Gain (Loss) in Statement of Financial Performance (Details) (USD $)
 3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative Instruments, Gain (Loss) Recognized in Income, Net $ (25,083,000) $ 7,286,000
Gain (Loss) on Interest Rate Swap and Swaption Agreements (16,193,000) 1,939,000
Interest Expense on Interest Rate Swap and Swaption Agreements (4,700,000) (3,200,000)
Termination of Interest Rate Swaps and Swaptions, Number of Positions 11 4
Termination of Interest Rate Swaps and Swaptions, Notional Amount 900,000,000 400,000,000
Termination of Interest Rate Swaps and Swaptions, Interest Spread Settlement (500,000) (400,000)
Gain on Termination of Interest Rate Swaps and Swaptions (11,265,000) 1,253,000
Termination of Credit Default Swaps, Number of Positions 4  
Termination of Credit Default Swaps, Notional Amount 85,000,000  
Termination of Credit Default Swaps, Interest Spread Settlement 10,492  
Loss on Termination of Credit Default Swaps (1,600,000)  
Interest Rate Swap [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative Liability, Average Notional Amount 6,366,593,000 1,300,000,000
Other Contract [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative Instruments, Gain (Loss) Recognized in Income, Net (2,637,000) (258,000)
Derivative Liability, Average Notional Amount 754,396,000  
Interest Rate Swaps and Swaptions Associated with U.S. Treasuries and TBA Contracts [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) on Interest Rate Swap and Swaption Agreements (1,648,000) (410,000)
Forward Sale Commitment [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative Instruments, Gain (Loss) Recognized in Income, Net 13,000 0
Derivative Liability, Average Notional Amount 0  
Interest Rate Swaps and Swaptions Associated with Repurchase Agreements [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) on Interest Rate Swap and Swaption Agreements (14,545,000) 2,349,000
Credit Default Swap, Buying Protection [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative Instruments, Gain (Loss) Recognized in Income, Net 8,220,000 2,338,000
Credit Default Swap, Selling Protection [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative Instruments, Gain (Loss) Recognized in Income, Net (24,301,000) 0
Inverse Interest-Only Securities [Member]
    
Derivative Instruments, Gain (Loss) [Line Items]    
Derivative Instruments, Gain (Loss) Recognized in Income, Net 9,815,000 3,267,000
Derivative Liability, Average Notional Amount $ 0