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WBX East Africa and Alpen (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
WBX East Africa and Alpen (Details) - USD ($) $ in Millions	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Schedule of Equity Method Investments
Equity method earnings (loss), net of tax	$ 0.0	$ (0.2)	$ (0.1)	$ (0.2)
Other current liabilities	442.0		442.0		$ 435.4
Other assets	$ 373.4		$ 373.4		360.0
Alpen Food Company South Africa (Pty) Limited
Schedule of Equity Method Investments
Equity Method Investment, Ownership Percentage	50.00%		50.00%
Alpen Food Company South Africa (Pty) Limited \| Equity Method Investments
Schedule of Equity Method Investments
Other assets	$ 3.5		$ 3.5		$ 3.6
Post Holdings, Inc. \| Weetabix East Africa Limited
Schedule of Equity Method Investments
Noncontrolling Interest, Ownership Percentage by Parent	50.10%		50.10%