Hourglass Sands (Condensed Statement of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Business Acquisition [Line Items]
Expenses	$ 75,463	$ 57,389	$ 157,812	$ 121,955
Net income	(3,344)	(23)	3,656	2,109
Hourglass Sands LLC [Member]
Business Acquisition [Line Items]
Revenue	42		105
Expenses	182	421	496	557
Net income	$ (140)	$ (421)	$ (391)	$ (557)