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Asset Retirement Obligation Liability and Corresponding Capital Asset (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2016
USD ($)
Asset Retirement Obligation Disclosure [Abstract]  
Asset retirement obligations, Beginning balance $ 4,704
Accretion expense 107
Additions 0
Settlements and reversals (665)
Asset retirement obligations, Ending balance $ 4,146