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Inventories (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Inventory, Net [Abstract]    
Raw materials $ 125.8 $ 106.2
Work-in-process 55.9 49.6
Finished goods 42.3 40.9
Operating and other supplies 139.0 120.8
Total inventories $ 363.0 $ 317.5