Reconciliation of Level 3 Fair Value Measurements (Details) - Derivative Financial Instrument Net Assets - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Roll-forward of Level 3 Assets
Fair Value, net assets, beginning of period	$ (8,834)	$ 2,316	$ (9,687)	$ (9,574)
Fair Value, net assets, end of period	(19,100)	8,619	(19,100)	8,619
Commodity Price Risk Management (loss), net
Roll-forward of Level 3 Assets
Changes in fair value included in statement of operations line item:	(4,701)	9,262	(6,854)	22,622
Settlements included in statement of operations line items:	(5,565)	(2,959)	(2,559)	(4,429)
Net change in fair value of unsettled derivatives included in statement of operations line item	$ (15,582)	$ 8,161	$ (9,412)	$ 17,194