Hourglass Sands (Condensed Statement of Operations) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Business Acquisition [Line Items]
Expenses	$ 82,349	$ 64,566
Net income	7,000	2,132
Hourglass Sands LLC [Member]
Business Acquisition [Line Items]
Revenue	63
Expenses	314	136
Net income	$ (251)	$ (136)

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