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, 2014
Mar. 31, 2014
Dec. 31, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2012
Derivative [Line Items]            
Derivative, Notional Amount $ 43,379,217 $ 43,576,146 $ 37,366,610 $ 44,262,605 $ 27,041,328 $ 22,377,656
Swap [Member]
            
Derivative [Line Items]            
Derivative, Notional Amount 6,000,000 10,000,000 10,000,000 19,000,000 0 0
Derivative, Fair Value, Net (772)   (3,773)      
Derivative, Cost 0   0      
Derivative, Unrealized Gains (Losses) (772)   (3,773)      
Derivative, Average Strike Swap Rate 0.633%   0.821%      
Derivative Financial Instruments, Assets [Member]
            
Derivative [Line Items]            
Derivative, Notional Amount 38,499,739   30,383,908      
Derivative, Fair Value, Net 331,601   549,859      
Derivative Financial Instruments, Assets [Member] | Swap [Member]
            
Derivative [Line Items]            
Derivative, Notional Amount 0   0      
Derivative, Fair Value, Net 0   0      
Determination Date, February 2014 [Member] | Swap [Member]
            
Derivative [Line Items]            
Derivative, Notional Amount     3,000,000      
Derivative, Fair Value, Net     625      
Derivative, Cost     0      
Derivative, Unrealized Gains (Losses)     625      
Derivative, Average Strike Swap Rate     0.768%      
Determination Date, March 2014 [Member] | Swap [Member]
            
Derivative [Line Items]            
Derivative, Notional Amount     5,000,000      
Derivative, Fair Value, Net     (3,171)      
Derivative, Cost     0      
Derivative, Unrealized Gains (Losses)     (3,171)      
Derivative, Average Strike Swap Rate     0.85%      
Determination Date, June 2014 [Member] | Swap [Member]
            
Derivative [Line Items]            
Derivative, Notional Amount     2,000,000      
Derivative, Fair Value, Net     (1,227)      
Derivative, Cost     0      
Derivative, Unrealized Gains (Losses)     (1,227)      
Derivative, Average Strike Swap Rate     0.828%      
Determination Date, September 2014 [Member] | Swap [Member]
            
Derivative [Line Items]            
Derivative, Notional Amount 6,000,000          
Derivative, Fair Value, Net (772)          
Derivative, Cost 0          
Derivative, Unrealized Gains (Losses) $ (772)          
Derivative, Average Strike Swap Rate 0.633%