Revenue (Summary of Contract Revenues by Type of Work) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation Of Revenue [Line Items]
Revenues	$ 184,811	$ 135,270	$ 377,448	$ 268,893
Type of Work [Member] | Operating Segment [Member] | Dredging [Member]
Disaggregation Of Revenue [Line Items]
Revenues	184,811	135,270	377,448	268,893
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Capital-U.S. [Member]
Disaggregation Of Revenue [Line Items]
Revenues	59,406	69,651	152,150	146,603
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Capital-Foreign [Member]
Disaggregation Of Revenue [Line Items]
Revenues	18,640	3,279	26,969	8,802
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Coastal Protection [Member]
Disaggregation Of Revenue [Line Items]
Revenues	58,195	38,121	91,938	79,982
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Maintenance [Member]
Disaggregation Of Revenue [Line Items]
Revenues	30,188	19,077	59,837	26,880
Type of Work [Member] | Operating Segment [Member] | Dredging [Member] | Rivers & Lakes [Member]
Disaggregation Of Revenue [Line Items]
Revenues	$ 18,382	$ 5,142	$ 46,554	$ 6,626