Quarterly report pursuant to Section 13 or 15(d)

Asset Retirement Obligations (Detail)

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Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Estimated fair value of the Company's asset retirement obligations    
Balances at January 1 $ 14,846 $ 11,463
Accretion expense 536 414
Additions 816 2,143
Revisions   (361)
Balance at September 30 $ 16,198 $ 13,659