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Cover - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2021
Feb. 04, 2022
Jun. 30, 2021
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2021    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 1-12001    
Entity Registrant Name ALLEGHENY TECHNOLOGIES INCORPORATED    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 25-1792394    
Entity Address, Address Line One 1000 Six PPG Place    
Entity Address, City or Town Pittsburgh,    
Entity Address, State or Province PA    
Entity Address, Postal Zip Code 15222-5479    
City Area Code 412    
Local Phone Number 394-2800    
Title of 12(b) Security Common stock, par value $0.10    
Trading Symbol ATI    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding (in shares)   127,447,661  
Entity Public Float     $ 2.7
Documents Incorporated by Reference
Documents Incorporated By Reference
Selected portions of the Proxy Statement for the Annual Meeting of Stockholders to be held on May 12, 2022 are incorporated by reference into Part III of this Report.
    
Amendment Flag false    
Entity Central Index Key 0001018963    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY