Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Related Party Transaction [Line Items]
Selling, general and administrative expenses	$ 396.0	$ 284.6	$ 216.3
Purchases from related party	62.2	8.6
Accounts payable	119.5	121.0
Master services agreement fees
Related Party Transaction [Line Items]
Selling, general and administrative expenses	3.0	3.4	$ 4.0
Post Holdings, Inc. | Accounts payable
Related Party Transaction [Line Items]
Accounts payable	$ 3.7	$ 1.5