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Fair Value of Financial Instruments (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Carrying value and fair value of long-term debt    
Carrying Value $ 250,000 $ 227,000
Fair Value 260,340 231,000
Fixed-rate bonds [Member]
   
Carrying value and fair value of long-term debt    
Carrying Value 250,000 200,000
Fair Value 260,340 204,000
Variable rate debt [Member]
   
Carrying value and fair value of long-term debt    
Carrying Value   27,000
Fair Value   $ 27,000