| Document and Entity Information | Aug. 16, 2021 |
| Cover [Abstract] | |
| Entity Registrant Name | ALLEGHENY TECHNOLOGIES INC |
| Amendment Flag | false |
| Entity Central Index Key | 0001018963 |
| Document Type | 8-K |
| Document Period End Date | Aug. 16, 2021 |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 1-12001 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, par value $0.10 per share |
| Trading Symbol | ATI |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
