Investments in Affiliates (Details 1) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Current assets	$ 17,235			$ 17,235		$ 8,080
Total oil and gas properties, net	133,698			133,698		36,862
Current liabilities	20,932			20,932		11,345
Non-current liabilities	92,399			92,399		32,168
Total members' equity	30,073			30,073		$ 14,655
Revenues	12,742	$ 4,195		25,363	$ 17,565
Operating expenses	515	(693)		(3,375)	3,518
Net income (loss)	(725)	(462)		2,264	$ 4,834
Carbon Appalachia [Member]
Schedule of Equity Method Investments [Line Items]
Current assets	22,478			22,478
Total oil and gas properties, net	81,833			81,833
Total other property and equipment, net	15,880			15,880
Other long-term assets	1,302			1,302
Current liabilities	15,313			15,313
Non-current liabilities	61,234			61,234
Total members' equity	44,946			44,946
Revenues	18,058	1,348	$ 2,905	62,480
Operating expenses	16,686	2,691	4,449	56,441
Income (loss) from operations	1,372	(1,344)	(1,544)	6,039
Net income (loss)	$ 537	$ (1,546)	$ (1,874)	$ 4,017