Investment in Laramie Energy, LLC - Summarized Financial Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
ASSETS
Current assets	$ 908,631		$ 586,592
Current liabilities	948,072		507,201
REVENUE
Income from operations	21,423	$ 27,656
Net income	61,092	15,185
Laramie Energy Company
ASSETS
Current assets	23,886		28,569
Non-current assets	771,958		788,515
Current liabilities	40,016		41,681
Non-current liabilities	276,420		$ 293,084
REVENUE
Natural gas and oil revenues	67,924	46,681
Income from operations	13,743	6,044
Net income	$ (2,983)	$ 7,290