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Other Long-Term Obligations and Commitments - Reconciliation of Liability for Plugging and Abandonment Costs (Detail)
$ in Thousands
6 Months Ended
Jun. 30, 2018
USD ($)
Commitments and Contingencies Disclosure [Abstract]  
Asset retirement obligation at beginning of period $ 23,578
Liabilities incurred 0
Liabilities settled (1,114)
Accretion expense 560
Asset retirement obligation at end of period $ 23,024