Hourglass Sands (Condensed Balance Sheet) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Current assets	$ 97,171	$ 71,637
Total assets	526,036	518,193
Total liabilities	268,651	268,870
Redeemable noncontrolling interests	4,000
Total liabilities, redeemable noncontrolling interests, and stockholders' equity	526,036	$ 518,193
Hourglass Sands, LLC [Member]
Business Acquisition [Line Items]
Current assets	5,054
Property and equipment	2,575
Total assets	7,629
Total liabilities	500
Redeemable noncontrolling interests	4,000
Members' equity	3,129
Total liabilities, redeemable noncontrolling interests, and stockholders' equity	$ 7,629