Summary of Significant Accounting Policies - Insurance Coverage (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($) policy item	Dec. 31, 2019 USD ($)
Insurance Coverage
Total insurance liability	$ 3.2	$ 3.7
Maritime employer's liability
Insurance Coverage
Primary limit of insurance coverage	10.0
Watercraft pollution policy
Insurance Coverage
Primary limit of insurance coverage	$ 5.0
Heavy Civil Marine Construction Segment
Insurance Coverage
Levels of insurance coverage maintained by the Company | item	5
Amount in excess of primary insurance coverage	$ 200.0
Primary limit of insurance coverage	$ 1.0
Commercial Concrete Segment
Insurance Coverage
Levels of insurance coverage maintained by the Company | item	5
Amount in excess of primary insurance coverage	$ 200.0
Primary limit of insurance coverage	$ 1.0
Number of employee health care insurance policies | policy	2