Impact of ASC 606 Adoption - Schedule of Impact of Adoption on Consolidated Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenues:
Natural gas sales	$ 181,462	$ 188,851
Oil sales	41,284	31,348
Other revenues	2,628
Total operating revenues	225,374	220,958
Expenses:
Production taxes	23,270	22,132
Gathering fees	23,055	20,929
Net income	47,493	$ (89,698)	$ 177,140
Accounting Standards Update 2014-09 | Under ASC 605
Revenues:
Natural gas sales	181,516
Oil sales	41,284
Other revenues	2,628
Total operating revenues	225,428
Expenses:
Production taxes	23,275
Gathering fees	23,061
Net income	47,536
Accounting Standards Update 2014-09 | Increase/ (Decrease)
Revenues:
Natural gas sales	(54)
Total operating revenues	(54)
Expenses:
Production taxes	(5)
Gathering fees	(6)
Net income	$ (43)