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Asset Retirement Obligations (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Dec. 31, 2013
Changes in the Company's asset retirement obligations      
Beginning asset retirement obligations $ 7,484    
Liabilities incurred during period 358    
Liabilities settled during period (22)    
Revisions in estimated cash flows 1,489    
Accretion expense 117 81  
Ending asset retirement obligations 9,426    
Less: current asset retirement obligations (407) [1]    
Long-term asset retirement obligations $ 9,019   $ 7,309
[1] Included in accrued liabilities in the Company’s unaudited condensed consolidated balance sheet at March 31, 2014.