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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Detail)
$ in Thousands
9 Months Ended
Sep. 30, 2015
USD ($)
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Balance at December 31, 2014 $ 42,549
Liabilities incurred during period 1,233
Liabilities settled during period (112)
Accretion expense during period 1,654 [1]
Revisions to estimates 241
Balance at September 30, 2015 $ 45,565
[1] Included in depreciation, depletion and amortization on the Company’s Condensed Consolidated Statement of Operations.