Revenue (Summary of Type of Customer, 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 Customer [Member] | Operating Segment [Member] | Dredging [Member]
Disaggregation Of Revenue [Line Items]
Revenues	169,914	167,920	347,547	385,615
Type of Customer [Member] | Operating Segment [Member] | Dredging [Member] | Federal Government [Member]
Disaggregation Of Revenue [Line Items]
Revenues	127,331	133,496	258,063	299,263
Type of Customer [Member] | Operating Segment [Member] | Dredging [Member] | State and Local Government [Member]
Disaggregation Of Revenue [Line Items]
Revenues	37,069	29,693	71,844	72,025
Type of Customer [Member] | Operating Segment [Member] | Dredging [Member] | Private [Member]
Disaggregation Of Revenue [Line Items]
Revenues	3,901	750	11,318	3,484
Type of Customer [Member] | Operating Segment [Member] | Dredging [Member] | Foreign [Member]
Disaggregation Of Revenue [Line Items]
Revenues	$ 1,613	$ 3,981	$ 6,322	$ 10,843