Hourglass Sands (Condensed Statement of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Business Acquisition [Line Items]
Expenses	$ 90,292	$ 77,397	$ 248,104	$ 199,352
Net income (loss)	(3,723)	2,914	(67)	5,023
Hourglass Sands LLC [Member]
Business Acquisition [Line Items]
Revenue	86	36	191	36
Expenses	133	350	629	907
Net income (loss)	$ (47)	$ (314)	$ (438)	$ (871)