Hourglass Sands (Condensed Balance Sheet) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Current assets	$ 83,262	$ 84,978
Total assets	508,811	515,499
Total liabilities	252,304	256,625
Total liabilities , redeemable noncontrolling interests, and stockholders' equity	508,811	515,499
Hourglass Sands LLC [Member]
Business Acquisition [Line Items]
Current assets	3,357	4,241
Property and equipment	3,049	3,092
Total assets	6,406	7,333
Total liabilities	13	502
Redeemable noncontrolling interests	4,000	4,000
Members' equity	2,393	2,831
Total liabilities , redeemable noncontrolling interests, and stockholders' equity	$ 6,406	$ 7,333