OPERATING SEGMENTS - Schedule of Disaggregated Revenue by Product Type (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Sales	$ 1,243.9	$ 1,117.4	$ 1,204.7
Loss on impairment of long-lived assets	53.7	193.4
Change in fair value of contingent consideration	(7.9)	(22.1)	0.0
Severance costs	4.3	15.8	5.5
Inventories
Segment Reporting Information [Line Items]
Tangible asset impairment charges		2.4
Fortress
Segment Reporting Information [Line Items]
Change in fair value of contingent consideration	7.9	22.1	0.8
Corporate & Other
Segment Reporting Information [Line Items]
Sales	15.1	28.6	21.8
Salt | Reportable Segments
Segment Reporting Information [Line Items]
Sales	1,022.5	907.8	1,010.8
Plant Nutrition | Reportable Segments
Segment Reporting Information [Line Items]
Sales	206.3	181.0	172.1
Highway Deicing Salt
Segment Reporting Information [Line Items]
Sales	642.7	546.4	641.7
Highway Deicing Salt | Corporate & Other
Segment Reporting Information [Line Items]
Sales	0.0	0.0	0.0
Highway Deicing Salt | Salt | Reportable Segments
Segment Reporting Information [Line Items]
Sales	642.7	546.4	641.7
Highway Deicing Salt | Plant Nutrition | Reportable Segments
Segment Reporting Information [Line Items]
Sales	0.0	0.0	0.0
Consumer & Industrial Salt
Segment Reporting Information [Line Items]
Sales	379.8	361.4	369.1
Consumer & Industrial Salt | Corporate & Other
Segment Reporting Information [Line Items]
Sales	0.0	0.0	0.0
Consumer & Industrial Salt | Salt | Reportable Segments
Segment Reporting Information [Line Items]
Sales	379.8	361.4	369.1
Consumer & Industrial Salt | Plant Nutrition | Reportable Segments
Segment Reporting Information [Line Items]
Sales	0.0	0.0	0.0
SOP
Segment Reporting Information [Line Items]
Sales	218.8	189.0	181.8
SOP | Corporate & Other
Segment Reporting Information [Line Items]
Sales	0.0	0.0	0.0
SOP | Salt | Reportable Segments
Segment Reporting Information [Line Items]
Sales	0.0	0.0	0.0
SOP | Plant Nutrition | Reportable Segments
Segment Reporting Information [Line Items]
Sales	218.8	189.0	181.8
Fire Retardant Products
Segment Reporting Information [Line Items]
Sales		14.2	8.6
Fire Retardant Products | Corporate & Other
Segment Reporting Information [Line Items]
Sales		14.2	8.6
Fire Retardant Products | Salt | Reportable Segments
Segment Reporting Information [Line Items]
Sales		0.0	0.0
Fire Retardant Products | Plant Nutrition | Reportable Segments
Segment Reporting Information [Line Items]
Sales		0.0	0.0
Revenue from Services
Segment Reporting Information [Line Items]
Sales		0.5	1.8
Revenue from Services | Corporate & Other
Segment Reporting Information [Line Items]
Sales		0.5	1.8
Revenue from Services | Salt | Reportable Segments
Segment Reporting Information [Line Items]
Sales		0.0	0.0
Revenue from Services | Plant Nutrition | Reportable Segments
Segment Reporting Information [Line Items]
Sales		0.0	0.0
Eliminations & Other
Segment Reporting Information [Line Items]
Sales	2.6	5.9	1.7
Eliminations & Other | Corporate & Other
Segment Reporting Information [Line Items]
Sales	15.1	13.9	11.4
Eliminations & Other | Salt | Reportable Segments
Segment Reporting Information [Line Items]
Sales	0.0	0.0	0.0
Eliminations & Other | Plant Nutrition | Reportable Segments
Segment Reporting Information [Line Items]
Sales	$ (12.5)	$ (8.0)	$ (9.7)