Accounts Receivable and Major Customers Revenue from Contracts with Customers (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Exploration and Production Revenue	$ 71,258	$ 30,073
Oil and Condensate Revenue	2,946	2,302
Natural Gas Production Revenue	2,790	2,343
Refining and Marketing Revenue	92
Gas Gathering, Transportation, Marketing and Processing Costs	3,359	2,551
Net Income (Loss) Available to Common Stockholders, Basic	10,295	$ 28,081
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Exploration and Production Revenue	71,258
Oil and Condensate Revenue	3,392
Natural Gas Production Revenue	2,790
Refining and Marketing Revenue	92
Gas Gathering, Transportation, Marketing and Processing Costs	3,805
Net Income (Loss) Available to Common Stockholders, Basic	10,295
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Exploration and Production Revenue	0
Oil and Condensate Revenue	446
Natural Gas Production Revenue	0
Refining and Marketing Revenue	0
Gas Gathering, Transportation, Marketing and Processing Costs	446
Net Income (Loss) Available to Common Stockholders, Basic	$ 0