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Document and Entity Information - USD ($)
 9 Months Ended
Sep. 30, 2017
Oct. 20, 2017
Jun. 30, 2016
Document And Entity Information [Abstract]      
Document Type 10-Q    
Document Period End Date Sep. 30, 2017    
Amendment Flag false    
Entity Registrant Name ALLEGHENY TECHNOLOGIES INCORPORATED    
Entity Central Index Key 0001018963    
Entity Current Reporting Status Yes    
Entity Voluntary Filers No    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Well-known Seasoned Issuer Yes    
Entity Common Stock, Shares Outstanding   108,863,099  
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus Q3    
Entity Public Float     $ 1,400,000,000