Basis of Presentation (Details) - USD ($) shares in Millions, $ in Millions					3 Months Ended		9 Months Ended
	Nov. 25, 2022	Aug. 11, 2022	Mar. 10, 2022	Mar. 09, 2022	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Sep. 30, 2022	Oct. 21, 2019
Post distribution of ownership, percent			80.10%
Distributions from Post Holdings, Inc. related to Spin-off				$ 550.4
Post distribution of ownership, shares	4.6	14.8	78.1
Payment of merger consideration			$ 115.5				$ 0.0	$ 115.5
Separation Costs | Selling, general and administrative expenses
Separation Costs					$ 0.0	$ 0.9	0.7	$ 13.2
7.00% Senior Notes Maturing in March 2030
Long-term debt, gross			$ 840.0		$ 840.0		$ 840.0		$ 840.0
Common Class A
Common stock, shares issued										39.4
Common Class A | Common Stock
Shares repurchased (in millions)								0.8
Common Class B
Voting power of common stock				67.00%
BellRing Common Stock | Common Stock
Shares repurchased (in millions)	0.9	0.8			1.3	0.1	4.0	0.1
BellRing Brands, LLC unit | BellRing Brands, Inc.
Common unit, issued										39.4
BellRing Brands, LLC unit | Post Holdings, Inc.
Common units, outstanding				97.5
Noncontrolling interest, ownership percentage by noncontrolling owners				71.50%
BellRing Common Stock | Post Holdings, Inc.
Common units, outstanding			19.4
BellRing Common Stock Ownership Percentage			14.20%						3.40%