| Document and Entity Information | Oct. 31, 2019 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 31, 2019 |
| Entity Registrant Name | Allegheny Technologies Incorporated |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-12001 |
| 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 |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001018963 |
| Title of 12(b) Security | Common Stock, par value $0.10 per share |
| Trading Symbol | ATI |
| Security Exchange Name | NYSE |
