Segment Information Reconciliation of Segment Information, Revenue and Other Operating Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue, Major Customer [Line Items]
Gain on Commodity Derivative Instruments	$ 18,005	$ 19,183	$ 78,752	$ 80,508
Other Operating Income	3,903	20,176	23,146	52,483
Total Revenue	397,126	287,185	1,294,979	978,180
Operating Segments [Member]
Revenue, Major Customer [Line Items]
Total Segment Revenue from Contracts with External Customers	$ 375,218	$ 247,826	$ 1,193,081	$ 845,189