Revenue from Contracts with Customers Revenue from Contracts with Customers (Revenue Impact of the Adoption of ASC606) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Adoption of ASC606 [Line Items]
Oil and natural gas revenues	$ 102,318	$ 83,173	$ 205,417	$ 170,771
Oil and natural gas operating costs	32,418	32,758	68,380	61,962
Oil and natural gas gross profit	69,900		137,037
Gas gathering and processing revenues	54,059	48,153	110,103	99,094
Deferred tax expense	2,029	6,379	5,636	20,315
Net income	8,150	$ 9,059	16,015	$ 24,988
Adjustments due to ASC606 [Member]
Adoption of ASC606 [Line Items]
Oil and natural gas revenues	(3,732)		(6,902)
Oil and natural gas operating costs	(3,732)		(6,902)
Oil and natural gas gross profit	0		0
Gas gathering and processing revenues	1,179		2,371
Deferred tax expense	289		581
Net income	890		1,790
Amounts without the Adoption of ASC606 [Member]
Adoption of ASC606 [Line Items]
Oil and natural gas revenues	106,050		212,319
Oil and natural gas operating costs	36,150		75,282
Oil and natural gas gross profit	69,900		137,037
Gas gathering and processing revenues	52,880		107,732
Deferred tax expense	1,740		5,055
Net income	$ 7,260		$ 14,225