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Asset Retirement Obligation (Details) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Dec. 31, 2013
Changes to the Company's asset retirement obligation liability      
Asset retirement obligation, beginning of period $ 48,536 $ 27,260  
Liabilities incurred 224 498  
Liabilities assumed 0 15,319  
Liabilities settled (2,902) (2,415)  
Accretion expense 2,223 1,203  
Revisions in estimated cash flows 743 987  
Asset retirement obligation, end of period 48,824 42,852  
Less: current portion of asset retirement obligation (1,426) (1,502) (3,113)
Long-term asset retirement obligation $ 47,398 $ 41,350 $ 45,423