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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Balance at December 31, 2014 $ 42,549us-gaap_AssetRetirementObligation
Liabilities incurred during period 372us-gaap_AssetRetirementObligationLiabilitiesIncurred
Liabilities settled during period (41)us-gaap_AssetRetirementObligationLiabilitiesSettled
Accretion expense during period 537us-gaap_AssetRetirementObligationAccretionExpense [1]
Revisions to estimates 241us-gaap_AssetRetirementObligationRevisionOfEstimate
Balance at March 31, 2015 $ 43,658us-gaap_AssetRetirementObligation
[1] Included in depreciation, depletion and amortization on the Company’s Condensed Consolidated Statement of Operations.