Summary of Significant Accounting Principles - Insurance Coverage (Details)	9 Months Ended
	Sep. 30, 2017 USD ($) levels policy	Dec. 31, 2016 USD ($)
Insurance Coverage [Line Items]
Number of insurance policies (policy) | policy	2
Accrual for insurance liabilities	$ 4,826,000	$ 5,223,000
Marine Construction
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 Construction | Other liability policies
Insurance Coverage [Line Items]
Primary limit of insurance coverage	1,000,000
Marine Construction | Maritime employer's liability
Insurance Coverage [Line Items]
Primary limit of insurance coverage	10,000,000
Marine Construction | Watercraft pollution policy
Insurance Coverage [Line Items]
Primary limit of insurance coverage	$ 5,000,000
Concrete Construction
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 Construction | Other liability policies
Insurance Coverage [Line Items]
Primary limit of insurance coverage	$ 1,000,000