Background (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions			12 Months Ended
	Mar. 10, 2022	Oct. 21, 2019	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022	Nov. 25, 2022	Aug. 11, 2022	Mar. 09, 2022	Sep. 30, 2021
Post distribution of ownership, percent	80.10%
Payment of merger consideration					$ 550.4
Post distribution of ownership, shares	78.1					4.6	14.8
Payments of Merger Related Costs, Financing Activities			$ 0.0	$ 0.0	115.5
Selling, general and administrative expenses | Separation costs
Separation Costs				$ 0.7	$ 14.5
Common Stock
Common stock, shares outstanding			128.8	131.2	135.3				39.5
7.00% Senior Notes Maturing in March 2030
Long-term Debt, Gross					$ 840.0
Old BellRing Stockholders
Common Stock, Per Share, Cash Paid	$ 2.97
Common Class A
Common stock, par value per share									$ 0.01
Issuance of common stock, shares		39.4
Common Class B
Common stock, par value per share									$ 0.01
Voting power of common stock								67.00%
BellRing Common Stock
Common stock, par value per share					$ 0.01
BellRing Common Stock | Post Holdings, Inc.
Common units, outstanding	19.4
BellRing Common Stock Ownership Percentage	14.20%
BellRing Brands, LLC unit | BellRing Brands, Inc.
Common unit, issued		39.4
Noncontrolling interest, ownership percentage by parent								28.50%
BellRing Brands, LLC unit | Post Holdings, Inc.
Noncontrolling interest, ownership percentage by noncontrolling owners								71.50%
Common units, outstanding								97.5