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Asset Retirement Obligation - Schedule of Asset Retirement Obligation (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Asset Retirement Obligation Disclosure [Abstract]    
Balance at January 1, $ 419 $ 4,114
Accretion   59
Payments (16) (1,905)
Downward revision   (1,000)
Gain (2) (286)
Balance at June 30, $ 401 $ 982