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Asset Retirement Obligations - Schedule of Changes in Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Asset retirement obligation — beginning of period $ 52,449 $ 48,799
Liabilities incurred during period 1,519 5,854
Liabilities settled during period (770) (4,944)
Accretion expense during period 2,939 2,657
Revisions to estimates 647 83
Asset retirement obligation — end of period 56,784 52,449
Current portion of total ARO balance $ 500 $ 100