Hourglass Sands (Condensed Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Current assets	$ 84,978	$ 67,686
Total assets	515,499	518,193
Total liabilities	256,625	268,870
Total liabilities , redeemable noncontrolling interests, and stockholders' equity	515,499	$ 518,193
Hourglass Sands LLC [Member]
Business Acquisition [Line Items]
Current assets	4,241
Property and equipment	3,092
Total assets	7,333
Total liabilities	502
Redeemable noncontrolling interests	4,000
Members' equity	2,831
Total liabilities , redeemable noncontrolling interests, and stockholders' equity	$ 7,333