Annual report pursuant to Section 13 and 15(d)

Derivative Instruments and Hedging Activities Schedule of Inverse Interest-Only Securities Reconciliation (Details)

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Derivative Instruments and Hedging Activities Schedule of Inverse Interest-Only Securities Reconciliation (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Derivatives, Fair Value [Line Items]    
Inverse Interest-only Securities, Accrued Interest $ 3,711  
Not Designated as Hedging Instrument [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative, Notional Amount 1,909,351 1,131,084
Derivative, Unamortized Premium 0 0
Derivative, Designated Credit Reserve 0 0
Derivative, Net, Unamortized (1,620,966) (973,066)
Derivative, Amortized Cost Basis 288,385 158,018
Derivative, Gross Unrealized Gains 21,616 4,606
Derivative, Gross Unrealized Losses (8,737) (7,385)
Derivative, Carrying Value $ 301,264 $ 155,239