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Asset Retirement Obligation (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Summary of asset retirement obligation activity    
Beginning balance $ 7,968 $ 9,717
Upward revision   611
Accretion 458 550
Payments (1,255) (1,199)
Ending Balance $ 6,374 $ 7,968