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Costs Incurred for Property Acquisition, Exploration and Development (Detail) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Acquisitions:      
Acreage purchases [1] $ 28,300   $ 62,075
Oil and gas properties [1] 1,683   18,269
Development [1] 484,186   1,177,526
Exploration:      
Exploration drilling costs incurred [1] 2,235   2,030
Gas gathering facilities:      
Development [1] 4,599   15,097
Subtotal [1] 536,221   1,328,659
Asset retirement obligations 19,561 [1] $ 3,855 20,245 [1]
Total costs incurred [1] 555,782   1,348,904
Expense      
Exploration:      
Exploration costs incurred [1] 14,096   50,920
Stock-based compensation expense [1] $ 1,122   $ 2,742
[1] Includes costs incurred whether capitalized or expensed.