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
Mar. 31, 2014
Dec. 31, 2013
Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount $ 10,000,000 $ 10,000,000
Derivative, Fair Value, Net 3,871 (3,773)
Derivative, Cost 0 0
Derivative, Unrealized Gains (Losses) 3,871 (3,773)
Derivative, Average Strike Swap Rate 0.793% 0.821%
Derivative Financial Instruments, Assets [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 45,175,522 30,383,908
Derivative, Fair Value, Net 429,419 549,859
Derivative Financial Instruments, Assets [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 10,000,000 0
Derivative, Fair Value, Net 3,871 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, May 2014 [Member]
    
Derivative [Line Items]    
Derivative, Fair Value, Net 872  
Determination Date, May 2014 [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 3,000,000  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) 872  
Derivative, Average Strike Swap Rate 0.67%  
Determination Date, June 2014 [Member]
    
Derivative [Line Items]    
Derivative, Fair Value, Net 2,167  
Determination Date, June 2014 [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 5,000,000 2,000,000
Derivative, Fair Value, Net   (1,227)
Derivative, Cost 0 0
Derivative, Unrealized Gains (Losses) 2,167 (1,227)
Derivative, Average Strike Swap Rate 0.846% 0.828%
Determination Date, September 2014 [Member] | Swap [Member]
    
Derivative [Line Items]    
Derivative, Notional Amount 2,000,000  
Derivative, Fair Value, Net 832  
Derivative, Cost 0  
Derivative, Unrealized Gains (Losses) $ 832  
Derivative, Average Strike Swap Rate 0.847%