Stockholders' Equity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2022	Jun. 30, 2022	Jun. 30, 2021	May 23, 2022	Mar. 10, 2022	Sep. 30, 2021	Oct. 21, 2019
Class of Stock [Line Items]
Total cost including broker’s commissions		$ 20.5	$ 0.0
Common stock, shares issued	136.4	136.4				39.5
Common stock, shares outstanding	136.3	136.3				39.5
Post distribution of ownership, shares					78.1
Share Repurchase Authorization				$ 50.0
Post Holdings, Inc.
Class of Stock [Line Items]
Common stock, shares issued					97.5
Old BellRing Shareholders
Class of Stock [Line Items]
Common stock, shares issued					38.9
Common Class A
Class of Stock [Line Items]
Common stock, shares issued							39.4
Common Class A | Common Stock
Class of Stock [Line Items]
Shares repurchased (in millions)		0.8
Average price per share		$ 23.36
Total cost including broker’s commissions		$ 18.1
BellRing Common Stock | Common Stock
Class of Stock [Line Items]
Shares repurchased (in millions)	0.1	0.1
Average price per share	$ 22.96	$ 22.96
Total cost including broker’s commissions	$ 2.4	$ 2.4