Revenue (Summary of Type of Customer, Contract Revenues) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Disaggregation Of Revenue [Line Items]
Revenues	$ 177,633	$ 217,695
Type of Customer [Member] | Operating Segment [Member] | Dredging [Member]
Disaggregation Of Revenue [Line Items]
Revenues	177,633	217,695
Type of Customer [Member] | Operating Segment [Member] | Dredging [Member] | Federal Government [Member]
Disaggregation Of Revenue [Line Items]
Revenues	130,732	165,767
Type of Customer [Member] | Operating Segment [Member] | Dredging [Member] | State and Local Government [Member]
Disaggregation Of Revenue [Line Items]
Revenues	34,775	42,332
Type of Customer [Member] | Operating Segment [Member] | Dredging [Member] | Private [Member]
Disaggregation Of Revenue [Line Items]
Revenues	7,417	2,734
Type of Customer [Member] | Operating Segment [Member] | Dredging [Member] | Foreign [Member]
Disaggregation Of Revenue [Line Items]
Revenues	$ 4,709	$ 6,862