Quarterly report pursuant to Section 13 or 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
Mar. 31, 2013
Dec. 31, 2012
Derivatives, Fair Value [Line Items]    
Inverse Interest-only Securities, Accrued Interest $ 3,633  
Not Designated as Hedging Instrument [Member]
   
Derivatives, Fair Value [Line Items]    
Derivative, Notional Amount 1,960,087 1,909,351
Derivative, Unamortized Premium 0 0
Derivative, Designated Credit Reserve 0 0
Derivative, Net, Unamortized (1,660,016) (1,620,966)
Derivative, Amortized Cost Basis 300,071 288,385
Derivative, Gross Unrealized Gains 19,432 21,616
Derivative, Gross Unrealized Losses (9,763) (8,737)
Derivative, Carrying Value $ 309,740 $ 301,264