Hourglass Sands (Condensed Balance Sheet) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Business Acquisition [Line Items]
Current assets	$ 81,286	$ 84,978
Total assets	512,895	515,499
Total liabilities	251,843	256,625
Total liabilities , redeemable noncontrolling interests, and stockholders' equity	512,895	515,499
Hourglass Sands LLC [Member]
Business Acquisition [Line Items]
Current assets	3,412	4,241
Property and equipment	3,053	3,092
Total assets	6,465	7,333
Total liabilities	25	502
Redeemable noncontrolling interests	4,000	4,000
Members' equity	2,440	2,831
Total liabilities , redeemable noncontrolling interests, and stockholders' equity	$ 6,465	$ 7,333