Hourglass Sands (Condensed Balance Sheet) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Current assets	$ 76,227	$ 84,978
Total assets	507,169	515,499
Total liabilities	242,042	256,625
Total liabilities , redeemable noncontrolling interests, and stockholders' equity	507,169	515,499
Hourglass Sands LLC [Member]
Business Acquisition [Line Items]
Current assets	3,889	4,241
Property and equipment	3,086	3,092
Total assets	6,975	7,333
Total liabilities	395	502
Redeemable noncontrolling interests	4,000	4,000
Members' equity	2,580	2,831
Total liabilities , redeemable noncontrolling interests, and stockholders' equity	$ 6,975	$ 7,333