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Cover - USD ($)
$ in Billions
 12 Months Ended
Dec. 31, 2023
Feb. 02, 2024
Jul. 02, 2023
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2023    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 1-12001    
Entity Registrant Name ATI Inc.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 25-1792394    
Entity Address, Address Line One 2021 McKinney Avenue    
Entity Address, City or Town Dallas,    
Entity Address, State or Province TX    
Entity Address, Postal Zip Code 75201    
City Area Code 800    
Local Phone Number 289-7454    
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    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Common Stock, Shares Outstanding (in shares)   127,781,255  
Entity Public Float     $ 5.7
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 16, 2024 are incorporated by reference into Part III of this Report.
    
Amendment Flag false    
Entity Central Index Key 0001018963    
Document Fiscal Year Focus 2023    
Document Fiscal Period Focus FY