Summary of Significant Accounting Policies - Insurance Coverage (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) item policy	Dec. 31, 2019 USD ($)
Insurance Coverage
Levels of insurance coverage maintained by the Company | item	5
Number of employee health care insurance policies | policy	2
Accrued insurance claims liability	$ 60,400	$ 3,700
Accrued insurance claims receivables	$ 57,021	$ 1,100
Heavy Civil Marine Construction Segment
Insurance Coverage
Levels of insurance coverage maintained by the Company | item	5
Amount in excess of primary insurance coverage	$ 200,000
Heavy Civil Marine Construction Segment | Other liability policies
Insurance Coverage
Primary limit of insurance coverage	1,000
Heavy Civil Marine Construction Segment | Maritime employer's liability
Insurance Coverage
Primary limit of insurance coverage	10,000
Heavy Civil Marine Construction Segment | Watercraft pollution policy
Insurance Coverage
Primary limit of insurance coverage	$ 5,000
Commercial Concrete Segment
Insurance Coverage
Levels of insurance coverage maintained by the Company | item	5
Amount in excess of primary insurance coverage	$ 200,000
Commercial Concrete Segment | Other liability policies
Insurance Coverage
Primary limit of insurance coverage	$ 1,000