GOODWILL AND INTANGIBLE ASSETS, NET - Rollforward of Carrying Amount of Goodwill by Reportable Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2025	Sep. 30, 2024
Goodwill [Line Items]
Goodwill	$ 862.7	$ 862.7	$ 862.7
Accumulated impairment losses	(618.8)	(618.8)	(618.8)
Goodwill, net	243.9	243.9	243.9
U.S. Consumer
Goodwill [Line Items]
Goodwill	245.7	245.7	245.7
Accumulated impairment losses	(1.8)	(1.8)	(1.8)
Goodwill, net	243.9	243.9	243.9
Hawthorne
Goodwill [Line Items]
Goodwill	617.0	617.0	617.0
Accumulated impairment losses	(617.0)	(617.0)	(617.0)
Goodwill, net	0.0	0.0	$ 0.0
Other non-reportable operating segment
Goodwill [Line Items]
Goodwill, net		$ 0.0
Impairment	$ (10.3)