Hourglass Sands (Condensed Statement of Operations) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Business Acquisition [Line Items]
Revenue		$ 79,722	$ 74,468	$ 203,829	$ 202,333
Expenses		77,397	70,542	199,352	189,621
Net income	[1]	$ 2,914	$ 3,916	5,023	$ 11,719
Hourglass Sands, LLC [Member]
Business Acquisition [Line Items]
Revenue				36
Expenses				907
Net income				$ (871)

[1] There is no material difference between net income and comprehensive income.