Business Combinations - Pro Forma Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
EPMH
Business Acquisition [Line Items]
Sales	$ 451,566	$ 341,113	$ 879,493	$ 629,196
Net income (loss)	$ 19,693	$ 13,196	$ 70,539	$ 8,097
Basic earnings per share (in dollars per share)	$ 0.25	$ 0.16	$ 0.90	$ 0.10
Diluted earnings per share (in dollars per share)	$ 0.25	$ 0.16	$ 0.89	$ 0.10
Basic shares (in shares)	77,784	81,087	78,636	81,032
Diluted shares (in shares)	78,480	81,945	79,328	82,103
MS Sand
Business Acquisition [Line Items]
Sales					$ 1,287,202	$ 642,951
Net income (loss)					$ 143,604	$ (55,835)
Basic earnings per share (in dollars per share)					$ 1.77	$ (0.86)
Diluted earnings per share (in dollars per share)					$ 1.75	$ (0.86)