Impact of ASC 606 Adoption - Schedule of Impact of Adoption on Condensed Consolidated Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenues:
Total operating revenues	$ 203,776	$ 217,631	$ 619,289	$ 651,247
Expenses:
Production taxes	20,470	22,482	62,623	66,369
Gathering fees	21,810	22,182	69,046	63,753
Net income	18,563	(327,727)	45,502	81,611	$ 177,140
Natural Gas Sales
Revenues:
Total operating revenues	156,986	182,949	479,704	551,797
Oil Sales
Revenues:
Total operating revenues	41,523	32,334	125,974	94,415
Other Revenues
Revenues:
Total operating revenues	$ 5,267	$ 2,348	13,611	$ 5,035
Accounting Standards Update 2014-09 | Increase/ (Decrease)
Revenues:
Total operating revenues			619,500
Expenses:
Production taxes			62,644
Gathering fees			69,072
Net income			45,666
Accounting Standards Update 2014-09 | Increase/ (Decrease)
Revenues:
Total operating revenues			(211)
Expenses:
Production taxes			(21)
Gathering fees			(26)
Net income			(164)
Accounting Standards Update 2014-09 | Natural Gas Sales | Increase/ (Decrease)
Revenues:
Total operating revenues			479,915
Accounting Standards Update 2014-09 | Natural Gas Sales | Increase/ (Decrease)
Revenues:
Total operating revenues			(211)
Accounting Standards Update 2014-09 | Oil Sales | Increase/ (Decrease)
Revenues:
Total operating revenues			125,974
Accounting Standards Update 2014-09 | Other Revenues | Increase/ (Decrease)
Revenues:
Total operating revenues			$ 13,611