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Asset Retirement Obligations - Asset Retirement Obligations (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation [Abstract]    
Beginning of period $ 312,754 $ 276,855
Liabilities incurred 4,063 3,376
Acquisitions 0 13,438
Liabilities settled (5,953) (5,052)
Disposition of wells (82,576) (13,332)
Accretion expense 15,658 25,456
Change in estimate 7,130 12,013
End of period 251,076 312,754
Less current portion (2,393) (5,485)
Long-term asset retirement obligations [1] $ 248,683 $ 307,269
[1] In fourth quarter 2019, we entered into an agreement to sell our legacy assets in Northwest Pennsylvania. Final approval from the state governmental authorities for a change in operatorship is expected by mid-2020, at which time we will reduce this liability by approximately $125.5 million and recognize a gain on sale of these assets