Annual report pursuant to Section 13 and 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
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Inverse Interest-Only Securities [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount $ 885,734 $ 176,064
Derivative Liability, Notional Amount 0 0
Interest Rate Swap [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 0 0
Derivative Liability, Notional Amount 3,667,493 377,400
Credit Default Swap [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 250,894 16,101
Derivative Liability, Notional Amount 135,952 0
Interest Rate Swaption [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 1,619,452 6,861
Derivative Liability, Notional Amount 0 0
Other Contract [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 313,630 75,609
Derivative Liability, Notional Amount 726,781 91,745
Forward Sale Commitment [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 128 0
Derivative Liability, Notional Amount $ 0 $ 0