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
Sep. 30, 2013
Dec. 31, 2012
Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount $ 8,000,000  
Derivative, Fair Value, Net (9,341)  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) (9,341)  
Derivative, Average Strike Swap Rate 0.882%  
Derivative Financial Instruments, Assets [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 27,657,236 9,768,656
Derivative, Fair Value, Net 544,515 462,080
Derivative Financial Instruments, Assets [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 0  
Derivative, Fair Value, Net 0  
Determination Date, November 2013 [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 3,000,000  
Derivative, Fair Value, Net (3,205)  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) (3,205)  
Derivative, Average Strike Swap Rate 0.869%  
Determination Date, December 2013 [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 5,000,000  
Derivative, Fair Value, Net (6,136)  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) $ (6,136)  
Derivative, Average Strike Swap Rate 0.89%