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Goodwill and Other Intangible Assets - Goodwill (Details)
 3 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2017
USD ($)
reporting_unit
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Goodwill [Line Items]            
Goodwill $ 531,400,000     $ 531,400,000 $ 641,900,000  
Period increase (decrease) in goodwill       (110,500,000)    
Impairment of goodwill   $ (114,400,000)   (114,400,000) 0 $ (126,600,000)
Accumulated goodwill impairment 241,000,000     241,000,000 126,600,000  
High Performance Materials & Components [Member]            
Goodwill [Line Items]            
Goodwill 531,400,000     531,400,000    
Impairment of goodwill $ 0     (114,400,000) $ 0  
Goodwill, Foreign Currency Translation Gain (Loss)       $ 3,900,000    
Number of reporting units | reporting_unit       2    
High Performance Materials & Components [Member] | First Reporting Unit [Member]            
Goodwill [Line Items]            
Percentage of fair value in excess of carrying value         12.00%  
Flat Rolled Products [Member]            
Goodwill [Line Items]            
Impairment of goodwill     $ (126,600,000)