Annual report pursuant to Section 13 and 15(d)

Schedule of Average Notional Amounts of Derivative Positions (Details)

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Schedule of Average Notional Amounts of Derivative Positions (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Inverse Interest-Only Securities [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount $ 1,782,306 $ 1,741,380  
Derivative Liability, Average Notional Amount 0 0  
Interest Rate Swap [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount 16,965,918 0  
Derivative Liability, Average Notional Amount 0 9,891,749 3,667,493
Credit Default Swap [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount 0 563,434  
Derivative Liability, Average Notional Amount 1,044,170 68,541  
Interest Rate Swaption [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount 5,592,274 4,023,639  
Derivative Liability, Average Notional Amount 0 0  
Other Contract [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount 2,186,016 546,429  
Derivative Liability, Average Notional Amount 1,508,836 526,085  
Options Held [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount 420,334 0  
Derivative Liability, Average Notional Amount 0 0  
Swap [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount 0 0  
Derivative Liability, Average Notional Amount 7,972,527 0  
Short US Treasury Securities [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount 0 0  
Derivative Liability, Average Notional Amount 6,658 11,749  
Forward Purchase Commitment [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount 57,489 94,855  
Derivative Liability, Average Notional Amount 0 0  
Forward Sale Commitment [Member]
      
Derivatives, Fair Value [Line Items]      
Derivative Asset, Average Notional Amount 0 2,395  
Derivative Liability, Average Notional Amount $ 0 $ 0