Revenue (Summary of Type of Work, Contract Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Revenues	$ 169,914	$ 167,920	$ 347,547	$ 385,615
Type of Work [Member] | Operating Segment [Member] | Dredging [Member]
Disaggregation Of Revenue [Line Items]
Revenues	169,914	167,920	347,547	385,615
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Capital-U.S. [Member]
Disaggregation Of Revenue [Line Items]
Revenues	79,399	63,440	157,005	146,989
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Capital-Foreign [Member]
Disaggregation Of Revenue [Line Items]
Revenues	1,613	3,981	6,322	10,843
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Coastal Protection [Member]
Disaggregation Of Revenue [Line Items]
Revenues	46,631	56,038	93,262	135,888
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Maintenance [Member]
Disaggregation Of Revenue [Line Items]
Revenues	37,278	41,968	82,579	84,353
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Rivers & Lakes [Member]
Disaggregation Of Revenue [Line Items]
Revenues	$ 4,993	$ 2,493	$ 8,379	$ 7,542