| Document and Entity Information - USD ($) $ in Millions | 12 Months Ended | ||
| Sep. 30, 2019 | Nov. 18, 2019 | Mar. 29, 2019 | |
| Cover page. | |||
| Document Type | 10-K/A | ||
| Document Annual Report | true | ||
| Document Transition Report | false | ||
| Entity File Number | 001-37793 | ||
| Entity Registrant Name | Atkore International Group Inc. | ||
| Entity Central Index Key | 0001666138 | ||
| Document Period End Date | Sep. 30, 2019 | ||
| Document Fiscal Year Focus | 2019 | ||
| Amendment Flag | false | ||
| Document Fiscal Period Focus | FY | ||
| Entity Well-known Seasoned Issuer | Yes | ||
| Entity Voluntary Filers | No | ||
| Entity Current Reporting Status | Yes | ||
| Entity Interactive Data Current | Yes | ||
| Entity Shell Company | false | ||
| Document Fiscal Year Focus | --09-30 | ||
| Entity Incorporation, State or Country Code | DE | ||
| 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 | ||
| Title of 12(b) Security | Common stock, par value $0.01 per share | ||
| Trading Symbol | ATKR | ||
| Security Exchange Name | NYSE | ||
| Entity Filer Category | Large Accelerated Filer | ||
| Entity Small Business | false | ||
| Entity Emerging Growth Company | false | ||
| Entity Common Stock, Shares Outstanding | 46,983,702 | ||
| Entity Public Float | $ 983.1 | ||
| Documents Incorporated by Reference | Portions of the registrant's proxy statement to be filed with the United States Securities and Exchange Commission in connection with the registrant's 2020 annual meeting of stockholders (the "Proxy Statement") are incorporated by reference into Part III hereof. Such Proxy Statement will be filed within 120 days of the registrant's fiscal year ended September 30, 2019 . |
