| Cover | Apr. 30, 2024 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Apr. 30, 2024 |
| Entity Registrant Name | ATI Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 1-12001 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.10 per share |
| Trading Symbol | ATI |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001018963 |
| Amendment Flag | false |
