Fair Value Measurements - Schedule of Fair Value on a Non-recurring Basis (Details) - USD ($) $ in Millions							3 Months Ended	12 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Sep. 03, 2024	Mar. 31, 2024	Mar. 22, 2024	Jan. 23, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of intangible assets					$ 15.6			$ 53.0	$ 15.6	$ 0.0
Loss on impairment of long-lived assets						$ 74.8		$ 0.7		$ 0.0
Goodwill, impairment loss							$ 83.0		83.0
Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Definitive-lived intangible assets		$ 0.0			0.0
Total Gains (Losses)		(53.0)	$ (17.6)		(15.6)
Indefinite-lived intangible assets			0.0
Property, plant and equipment remaining	$ 0.0					0.0
Gain on disposition of property plant equipment	(0.7)					(74.8)
Water rights
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of intangible assets			17.6
Plant Nutrition
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, impairment loss				$ 51.0			51.0		$ 51.0
Plant Nutrition | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, fair value				0.0			0.0
Gain (Loss) on disposition of goodwill				(51.0)
Fortress
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Property, plant and equipment remaining	0.0
Impairment of intangible assets		53.0
Property, plant and equipment, net	0.7
Loss on impairment of long-lived assets	0.7
Goodwill, impairment loss				32.0			32.0
Fortress | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, fair value				0.0			0.0
Gain (Loss) on disposition of goodwill				(32.0)
Quoted Prices in Active Market (Level One) | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Definitive-lived intangible assets		0.0			0.0
Indefinite-lived intangible assets			0.0
Property, plant and equipment remaining	0.0					0.0
Quoted Prices in Active Market (Level One) | Plant Nutrition | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, fair value				0.0			0.0
Quoted Prices in Active Market (Level One) | Fortress | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, fair value				0.0			0.0
Significant Other Observable Inputs (Level Two) | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Definitive-lived intangible assets		0.0			0.0
Indefinite-lived intangible assets			0.0
Property, plant and equipment remaining	0.0					0.0
Significant Other Observable Inputs (Level Two) | Plant Nutrition | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, fair value				0.0			0.0
Significant Other Observable Inputs (Level Two) | Fortress | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, fair value				0.0			0.0
Significant Unobservable Inputs (Level Three) | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Definitive-lived intangible assets		$ 0.0			$ 0.0
Indefinite-lived intangible assets			$ 0.0
Property, plant and equipment remaining	$ 0.0					$ 0.0
Significant Unobservable Inputs (Level Three) | Plant Nutrition | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, fair value				0.0			0.0
Significant Unobservable Inputs (Level Three) | Fortress | Fair Value, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Goodwill, fair value				$ 0.0			$ 0.0