Property And Equipment - Gross Components Of Property And Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment	$ 2,948,907	$ 2,922,274
Vessels
Property, Plant and Equipment [Line Items]
Property and equipment	$ 2,349,752	2,323,314
Vessels | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years
Vessels | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	30 years
ROVs, Trenchers and ROVDrill
Property, Plant and Equipment [Line Items]
Property and equipment	$ 263,968	270,004
Estimated useful life	10 years
Machinery, Equipment and Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment	$ 335,187	$ 328,956
Machinery, Equipment and Leasehold Improvements | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Machinery, Equipment and Leasehold Improvements | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years