| Document and Entity Information Document | Jun. 03, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jun. 3, 2020 |
| Entity Registrant Name | Atkore International Group Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-37793 |
| Entity Tax Identification Number | 90-0631463 |
| Entity Address, Address Line One | 16100 South Lathrop Avenue |
| Entity Address, City or Town | Harvey |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60426 |
| City Area Code | 708 |
| Local Phone Number | 339-1610 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $.01 par value per share |
| Trading Symbol | ATKR |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001666138 |
| Amendment Flag | false |
