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Inventories (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Inventory, Net [Abstract]    
Raw materials $ 108.1 $ 106.2
Work-in-process 54.3 49.6
Finished goods 41.7 40.9
Operating and other supplies 126.6 120.8
Total inventories $ 330.7 $ 317.5