Related Party Transactions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021	Mar. 10, 2022	Mar. 09, 2022	Sep. 30, 2021
Related Party Transaction [Line Items]
Common stock, shares outstanding	136,400,000		136,400,000				39,500,000
Cash distributions directly to related parties			$ 3.2	$ 9.3
Cash distributions on behalf of related parties to third party				$ 1.4
Percentage of tax savings owed to related party		85.00%		85.00%
Common Class B
Related Party Transaction [Line Items]
Common stock, shares outstanding						1
Voting power of common stock						67.00%
Post Holdings, Inc. | BellRing Brands, LLC unit
Related Party Transaction [Line Items]
Common units, outstanding						97,500,000	97,500,000
Noncontrolling interest, ownership percentage by noncontrolling owners						71.50%	71.20%
Post Holdings, Inc. | BellRing Common Stock
Related Party Transaction [Line Items]
BellRing Common Stock Ownership Percentage	14.20%		14.20%		14.20%
Accounts payable
Related Party Transaction [Line Items]
Accounts payable, trade, related parties	$ 2.7		$ 2.7				$ 2.2
Due to Related Parties							0.3
Tax receivable agreement, related parties	0.4		0.4
Other liabilities
Related Party Transaction [Line Items]
Due to Related Parties							$ 10.2
Master services agreement fees | Selling, general and administrative expenses
Related Party Transaction [Line Items]
Expenses from transactions with related party	1.2	$ 0.5	1.8	$ 1.1
Share-based payment arrangement | Selling, general and administrative expenses
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ 0.3	$ 0.6	$ 0.8	$ 1.4