Summary Of Major Accounting Policies - Property, Plant and Equipment and Long-Lived Intangible Assets (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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		$ 7.3	$ 4.6	$ 3.7
Amortization of Intangible Assets		$ 17.4	$ 10.2	$ 10.2
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		25 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