Summary of Significant Accounting Principles - Goodwill and Other Intangible Assets (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Goodwill [Line Items]
Goodwill	$ 69,483	$ 69,483	$ 66,351
Number of reportable segments (segment) | segment	2
Number of operating segments (segment) | segment	2
Marine
Goodwill [Line Items]
Goodwill	$ 33,800
Concrete
Goodwill [Line Items]
Goodwill	$ 35,700