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Document and Entity Information - USD ($)
$ in Billions
 3 Months Ended
Mar. 31, 2019
Apr. 19, 2019
Jun. 30, 2018
Document And Entity Information [Abstract]      
Document Type 10-Q    
Document Period End Date Mar. 31, 2019    
Amendment Flag false    
Entity Registrant Name ALLEGHENY TECHNOLOGIES INCORPORATED    
Entity Central Index Key 0001018963    
Entity Current Reporting Status Yes    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Common Stock, Shares Outstanding   126,034,880  
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus Q1    
Entity Public Float     $ 3.2