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Acquisitions Acquisitions - Bluewater - Purchase Price Allocation (Details) - USD ($)
$ in Millions
1 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Business Acquisition [Line Items]      
Goodwill   $ 7.3 $ 0.0
Bluewater      
Business Acquisition [Line Items]      
Current assets $ 2.0    
Property, plant, and equipment, net 217.6    
Goodwill 7.3    
Current liabilities (0.9)    
Total purchase price $ 226.0