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Asset Retirement Obligation (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Changes to the Company's asset retirement obligation liability    
Asset retirement obligation, beginning of period $ 27,260 $ 30,427
Liabilities assumed 15,319 0
Liabilities incurred 498 892
Liabilities settled (3,335) (2,627)
Accretion expense 1,753 2,078
Revisions in estimated cash flows 7,041 (3,510)
Asset retirement obligation, end of period 48,536 27,260
Less: current portion of asset retirement obligation (3,113) (2,351)
Long-term asset retirement obligation $ 45,423 $ 24,909