Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities Schedule of Average Notional Amounts of Derivative Positions (Details)

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Derivative Instruments and Hedging Activities Schedule of Average Notional Amounts of Derivative Positions (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2012
Inverse Interest-Only Securities [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount $ 1,953,128 $ 1,669,944
Derivative Liability, Average Notional Amount 0 0
Interest Rate Swap [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 0 0
Derivative Liability, Average Notional Amount 11,327,391 8,625,967
Credit Default Swap [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 545,611 554,568
Derivative Liability, Average Notional Amount 48,576 91,555
Interest Rate Swaption [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 4,891,304 3,687,328
Derivative Liability, Average Notional Amount 0 0
Other Contract [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 304,348 278,741
Derivative Liability, Average Notional Amount 0 564,416
Short US Treasury Securities [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 0 0
Derivative Liability, Average Notional Amount 46,739 15,693
Forward Purchase Commitment [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 45,872 15,894
Derivative Liability, Average Notional Amount 0 0
Forward Sale Commitment [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 0 3,199
Derivative Liability, Average Notional Amount $ 0 $ 0