Document and Entity Information - shares	3 Months Ended
	Dec. 29, 2017	Jan. 26, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	Atkore International Group Inc.
Entity Central Index Key	0001666138
Document Type	10-Q
Document Period End Date	Dec. 29, 2017
Document Fiscal Year Focus	2018
Amendment Flag	false
Document Fiscal Period Focus	Q1
Document Fiscal Year Focus	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		63,607,047