Restructuring Charges (Schedule of Restructuring Charges) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Restructuring Cost And Reserve [Line Items]
Restructuring charges	$ 534	$ 6,808
Loss on Sale of Assets - Net [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	(840)	(847)
Other Expense [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges		2,015
Dredging [Member] | Costs of Contract Revenues - Depreciation [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	1,676	4,668
Dredging [Member] | Costs of Contract Revenues - Other [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	(274)	993
Dredging [Member] | General and Administrative Expenses [Member]
Restructuring Cost And Reserve [Line Items]
Restructuring charges	$ (28)	$ (21)