Investment in Laramie Energy, LLC - Summarized Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
ASSETS
Current assets	$ 951,033				$ 951,033		$ 586,592
Current liabilities	974,193				974,193		507,201
REVENUE
Income (loss) from operations	48,621		$ 28,983		70,044	$ 56,639
Net income	28,169	$ 61,092	16,178	$ 15,185	89,261	31,363
Laramie Energy Company
ASSETS
Current assets	21,028				21,028		28,569
Non-current assets	769,913				769,913		788,515
Current liabilities	33,986				33,986		41,681
Non-current liabilities	280,047				280,047		$ 293,084
REVENUE
Natural gas and oil revenues	43,270		46,750		111,194	93,431
Income (loss) from operations	(8,205)		(554)		5,538	5,490
Net income	$ (2,570)		$ (8,846)		$ (5,553)	$ (1,556)