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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
9 Months Ended
Sep. 30, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Balance at December 31, 2012 $ 23,234
Liabilities incurred during period 3,066
Liabilities settled during period 23
Accretion expense during period 895 [1]
Revisions to estimates 213
Balance at September 30, 2013 $ 27,431
[1] Included in depreciation, depletion and amortization on the Company’s Condensed Consolidated Statement of Operations.