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
Mar. 31, 2013
Mar. 31, 2012
Inverse Interest-Only Securities [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount $ 1,922,195  
Derivative Liability, Average Notional Amount 0  
Interest Rate Swap [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 0  
Derivative Liability, Average Notional Amount 14,854,500 6,366,593
Credit Default Swap [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 437,845  
Derivative Liability, Average Notional Amount 0  
Interest Rate Swaption [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 5,542,778  
Derivative Liability, Average Notional Amount 0  
Other Contract [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 1,213,644  
Derivative Liability, Average Notional Amount 326,667  
Forward Purchase Commitment [Member]
    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Notional Amount 51,783  
Derivative Liability, Average Notional Amount $ 0