Hourglass Sands (Condensed Statement of Operations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Revenue	$ 293,557	$ 271,633
Expenses	290,011	257,751
Net income	7,621	$ 33,076
Hourglass Sands LLC [Member]
Business Acquisition [Line Items]
Revenue	255
Expenses	1,424
Net income	$ (1,169)