Summary of Significant Accounting Policies - Insurance Coverage (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2021 USD ($) item policy	Dec. 31, 2020 USD ($)
Insurance Coverage
Number of employee health care insurance policies | policy	2
Accrued insurance claims liability	$ 68,800	$ 60,400
Accrued insurance claims receivables	$ 64,726	$ 57,021
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