Schedule I - Condensed Financial Information - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Cash Flows from Operating Activities:
Net cash provided by operating activities	$ 209,694	$ 145,703	$ 121,654
Cash Flows from Investing Activities:
Net cash (used for) provided by investing activities	(133,101)	2,514	(205,833)
Cash Flows from Financing Activities:
Issuance of common stock	7,374	20,110	12,168
Repurchase of common stock	(24,419)	(411,775)	(13,938)
Net cash used for financing activities	(78,180)	(65,931)	(67,760)
Increase (decrease) in cash and cash equivalents	(3,247)	80,944	(154,561)
Atkore International Group Inc.
Cash Flows from Operating Activities:
Net cash provided by operating activities	0	0	0
Cash Flows from Investing Activities:
Distribution received from subsidiary	24,419	411,775	13,938
Distribution paid to subsidiary	(7,374)	(20,110)	(12,168)
Net cash (used for) provided by investing activities	17,045	391,665	1,770
Cash Flows from Financing Activities:
Issuance of common stock		20,110	12,168
Repurchase of common stock	(24,419)	(411,775)	(13,938)
Net cash used for financing activities	(17,045)	(391,665)	(1,770)
Increase (decrease) in cash and cash equivalents	0	0	0
Cash and cash equivalents:
Cash and cash equivalents at beginning of period	0	0	0
Cash and cash equivalents at end of period	$ 0	$ 0	$ 0