Revenue (Summary of Type of Work, Contract Revenues) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Contract revenues	$ 620,795	$ 592,159	$ 637,468
Type of Work [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	620,795	592,159	637,468
Type of Work [Member] | Dredging [Member] | Capital-U.S. [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	333,037	185,113	219,914
Type of Work [Member] | Dredging [Member] | Capital-Foreign [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	14,088	42,306	59,413
Type of Work [Member] | Dredging [Member] | Coastal Protection [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	175,923	191,070	215,041
Type of Work [Member] | Dredging [Member] | Maintenance [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	53,427	134,923	92,274
Type of Work [Member] | Dredging [Member] | Rivers & Lakes [Member]
Disaggregation Of Revenue [Line Items]
Contract revenues	$ 44,320	$ 38,747	$ 50,826