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 6 Months Ended
Jun. 30, 2012
Jun. 30, 2012
Inverse Interest-Only Securities [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount $ 1,727,601 $ 1,529,218
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 8,154,231 7,300,525
Credit Default Swap [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 570,354 556,121
Derivative Liability, Notional Amount 88,077 112,822
Interest Rate Swaption [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 3,790,110 3,307,792
Derivative Liability, Notional Amount 0 0
Other Contract [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 324,176 267,265
Derivative Liability, Notional Amount 945,055 854,420
Forward Purchase Commitment [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 1,480 601
Derivative Liability, Notional Amount 0 0
Forward Sale Commitment [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 4,446 4,816
Derivative Liability, Notional Amount   $ 0