Summary of Significant Accounting Principles - Insurance Coverage (Details)	9 Months Ended
	Sep. 30, 2018 USD ($) levels policy	Dec. 31, 2017 USD ($)
Insurance Coverage [Line Items]
Number of insurance policies (policy) | policy	2
Accrual for insurance liabilities	$ 5,492,000	$ 5,233,000
Marine
Insurance Coverage [Line Items]
Levels of insurance coverage maintained by the Company | levels	5
Amount in excess of primary insurance coverage	$ 200,000,000
Marine | Other liability policies
Insurance Coverage [Line Items]
Primary limit of insurance coverage	1,000,000
Marine | Maritime employer's liability
Insurance Coverage [Line Items]
Primary limit of insurance coverage	10,000,000
Marine | Watercraft pollution policy
Insurance Coverage [Line Items]
Primary limit of insurance coverage	$ 5,000,000
Concrete
Insurance Coverage [Line Items]
Levels of insurance coverage maintained by the Company | policy	5
Amount in excess of primary insurance coverage	$ 200,000,000
Concrete | Other liability policies
Insurance Coverage [Line Items]
Primary limit of insurance coverage	$ 1,000,000