Basis of Presentation Per Share (Details) - $ / shares		6 Months Ended
	Mar. 10, 2022	Mar. 31, 2022	Mar. 09, 2022	Sep. 30, 2021	Oct. 21, 2019
Post distribution of ownership, percent	80.10%
Common stock, shares outstanding		136,400,000		39,500,000
Post distribution of ownership, shares	78,100,000
Common stock, shares issued				39,500,000
Common Class B
Common stock, par value per share		$ 0.01
Common stock, shares outstanding			1
Voting power of common stock			67.00%
Common Class A
Common stock, par value per share		0.01
Common stock, shares issued					39,400,000
Common Stock
Common stock, par value per share		$ 0.01
Post Holdings, Inc.
Common stock, shares issued	97,500,000
Post Holdings, Inc. | BellRing Common Stock
BellRing Common Stock Ownership Percentage	14.20%	14.20%
Post Holdings, Inc. | BellRing Brands, LLC unit
Noncontrolling interest, ownership percentage by noncontrolling owners			71.50%	71.20%
Common units, outstanding			97,500,000	97,500,000
Post Shareholders | BellRing Common Stock
Conversion of Stock, Shares Received		1.267788
Post Shareholders | BellRing Common Stock
BellRing Common Stock Ownership Percentage	57.30%
Old BellRing Shareholders
Old BellRing Class A Common Stock, Cash Dividend Per Share	$ 2.97
Common stock, shares issued	38,900,000
Old BellRing Shareholders | BellRing Common Stock
BellRing Common Stock Ownership Percentage	28.50%
BellRing Brands, Inc. | BellRing Brands, LLC unit
Common unit, issued					39,400,000