Background (Details) - USD ($) $ / shares in Units, $ in Millions				12 Months Ended
	Aug. 11, 2022	Mar. 10, 2022	Oct. 21, 2019	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	Mar. 09, 2022	Sep. 30, 2019
Post distribution of ownership, percent		80.10%
Payment of merger consideration				$ 550.4
Post distribution of ownership, shares	14,800,000	78,100,000
Payments of Merger Related Costs, Financing Activities				115.5	$ 0.0	$ 0.0
Selling, general and administrative expenses | Separation costs
Expenses from transactions with related party				$ 14.5	$ 0.2	$ 1.9
Common Stock
Issuance of common stock, shares				0	0	39,400,000
Common stock, shares outstanding				135,300,000	39,500,000	39,400,000		0
Purchases of treasury stock, shares				(1,900,000)	0	0
7.00% Senior Notes Maturing in March 2030
Long-term Debt, Gross				$ 840.0
Old BellRing Stockholders
Common stock, shares outstanding					39,500,000
Common Stock, Per Share, Cash Paid		$ 2.97
Common Class A
Common stock, par value per share				$ 0.01	$ 0.01
Issuance of common stock, shares			39,400,000
Common stock, shares outstanding				0	39,510,430
Common Class A | Common Stock
Purchases of treasury stock, shares	800,000			800,000
Common Class B
Common stock, par value per share					$ 0.01
Voting power of common stock							67.00%
Common stock, shares outstanding				0	1
BellRing Common Stock
Common stock, par value per share				$ 0.01
BellRing Common Stock | Common Stock
Purchases of treasury stock, shares				1,100,000
BellRing Common Stock | Post Shareholders
Conversion of Stock, Shares Received				1.267788
BellRing Brands, LLC unit | BellRing Brands, Inc.
Common unit, issued			39,400,000
BellRing Brands, LLC unit | Post Holdings, Inc.
Noncontrolling interest, ownership percentage by noncontrolling owners					71.20%	71.20%	71.50%
Common units, outstanding		19,400,000			97,500,000	97,500,000	97,500,000
BellRing Common Stock | Post Holdings, Inc.
Noncontrolling interest, ownership percentage by noncontrolling owners				3.40%
BellRing Common Stock Ownership Percentage		14.20%
BellRing Common Stock | Post Shareholders
BellRing Common Stock Ownership Percentage		57.30%
BellRing Common Stock | Old BellRing Stockholders
BellRing Common Stock Ownership Percentage		28.50%			28.80%	28.80%