Revenue from Contracts with Customers (Revenue Impact of the Adoption of ASC606) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Adoption of ASC606 [Line Items]
Oil and natural gas revenues	$ 111,623	$ 85,470	$ 317,040	$ 256,241
Oil and natural gas operating costs	32,139	33,911	100,519	95,873
Oil and natural gas gross profit	79,484		216,521
Gas gathering and processing revenues	57,823	51,399	167,926	150,493
Deferred tax expense	6,744	1,769	12,380	22,084
Net income	21,123	$ 3,705	37,138	$ 28,693
Adjustments due to ASC606 [Member]
Adoption of ASC606 [Line Items]
Oil and natural gas revenues	(5,200)		(12,102)
Oil and natural gas operating costs	(5,200)		(12,102)
Oil and natural gas gross profit	0		0
Gas gathering and processing revenues	1,300		3,671
Deferred tax expense	318		899
Net income	982		2,772
Amounts without the Adoption of ASC606 [Member]
Adoption of ASC606 [Line Items]
Oil and natural gas revenues	116,823		329,142
Oil and natural gas operating costs	37,339		112,621
Oil and natural gas gross profit	79,484		216,521
Gas gathering and processing revenues	56,523		164,255
Deferred tax expense	6,426		11,481
Net income	$ 20,141		$ 34,366