Restructuring Charges (Schedule of Restructuring Charges) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Restructuring Cost And Reserve [Line Items]
Restructuring charges	$ 29,485
Costs of Contract Revenues - Depreciation [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	6,859
Costs of Contract Revenues - Other [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	16,109
General and Administrative Expenses [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	1,826
Loss on Sale of Assets—Net [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	4,691
Dredging [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	28,841
Dredging [Member] | Costs of Contract Revenues - Depreciation [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	6,859
Dredging [Member] | Costs of Contract Revenues - Other [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	16,102
Dredging [Member] | General and Administrative Expenses [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	1,189
Dredging [Member] | Loss on Sale of Assets—Net [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	4,691
Environmental And Infrastructure [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	644
Environmental And Infrastructure [Member] | Costs of Contract Revenues - Other [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	7
Environmental And Infrastructure [Member] | General and Administrative Expenses [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	$ 637