Summary Of Major Accounting Policies - Property, Plant and Equipment and Long-Lived Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Acquired Finite-lived Intangible Asset, Weighted Average Useful Life	8 years
Property, Plant and Equipment [Line Items]
Interest Costs, Capitalized, Minimum Construction Period	3 months
Interest Costs, Capitalized During Period	$ 4.6	$ 3.7	$ 2.4
Amortization of Intangible Assets	$ 10.2	$ 10.2	$ 7.8
Remotely Operated Vehicles [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Average	8 years
Maximum [Member]
Property, Plant and Equipment [Line Items]
Cash and Cash Equivalents, Short-term Investments, Maturity Period	3 months
Maximum [Member] | Marine Services Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Average	20 years
Maximum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Average	25 years
Minimum [Member] | Marine Services Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Average	3 years
Minimum [Member] | Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life, Average	3 years