Property And Equipment - Gross Components Of Property And Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment	$ 2,695,772	$ 2,450,890
Vessels [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 2,083,267	1,860,379
Vessels [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years
Vessels [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	30 years
ROVs, trenchers and ROVDrills [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 298,227	309,603
Estimated useful life	10 years
Machinery, equipment and leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment	$ 314,278	$ 280,908
Machinery, equipment and leasehold improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Machinery, equipment and leasehold improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life	15 years