Quarterly report pursuant to Section 13 or 15(d)

Derivative Instruments and Hedging Activities Schedule of Constant Maturity Swaps (Details)

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Derivative Instruments and Hedging Activities Schedule of Constant Maturity Swaps (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount $ 19,000,000  
Derivative, Fair Value, Net (14,058)  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) (14,058)  
Derivative, Average Strike Swap Rate 0.892%  
Derivative Financial Instruments, Assets [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 29,556,972 9,768,656
Derivative, Fair Value, Net 699,351 462,080
Derivative Financial Instruments, Assets [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 0  
Derivative, Fair Value, Net 0  
Determination Date, August 2013 [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 8,000,000  
Derivative, Fair Value, Net (3,497)  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) (3,497)  
Derivative, Average Strike Swap Rate 0.837%  
Determination Date, September 2013 [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 5,000,000  
Derivative, Fair Value, Net (6,527)  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) (6,527)  
Derivative, Average Strike Swap Rate 0.981%  
Determination Date, November 2013 [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 1,000,000  
Derivative, Fair Value, Net (670)  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) (670)  
Derivative, Average Strike Swap Rate 0.90%  
Determination Date, December 2013 [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 5,000,000  
Derivative, Fair Value, Net (3,364)  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) $ (3,364)  
Derivative, Average Strike Swap Rate 0.89%