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Property, Plant and Equipment, net - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Property, Plant and Equipment [Line Items]          
Depreciation expense $ 11.9 $ 9.1 $ 34.2 $ 25.9  
Depreciation expense with cost of revenue 1.8 $ 1.3 5.1 $ 3.8  
Cable-ships and submersibles          
Property, Plant and Equipment [Line Items]          
Net book value of equipment under capital leases $ 40.6   $ 40.6   $ 45.3