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Fair Value of Financial Instruments (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Carrying value and fair value of long-term debt    
Carrying Value $ 250,000 $ 250,000
Fair Value 263,750 260,340
Fixed-rate bonds [Member]
   
Carrying value and fair value of long-term debt    
Carrying Value 250,000 250,000
Fair Value 263,750 260,340
Variable rate debt [Member]
   
Carrying value and fair value of long-term debt    
Carrying Value      
Fair Value