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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Detail) (USD $)
In Thousands, unless otherwise specified
6 Months Ended
Jun. 30, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]  
Balance at December 31, 2012 $ 23,234
Liabilities incurred during period 2,635
Liabilities settled during period 23
Accretion expense during period (1) 581 [1]
Revisions to estimates 227
Balance at June 30, 2013 $ 26,700
[1] Included in depreciation, depletion and amortization on the Company’s Condensed Consolidated Statement of Operations.