XML 12 R1.htm IDEA: XBRL DOCUMENT v3.10.0.1
Document and Entity Information - USD ($)
 9 Months Ended
Sep. 30, 2018
Oct. 19, 2018
Jun. 30, 2017
Document And Entity Information [Abstract]      
Document Type 10-Q    
Document Period End Date Sep. 30, 2018    
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   125,684,396  
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus Q3    
Entity Public Float     $ 1,900,000,000