Stockholders' Equity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 10, 2022	Sep. 30, 2021	Oct. 21, 2019
Class of Stock [Line Items]
Total cost including broker’s commissions	$ 18.1	$ 0.0
Common stock, shares issued				39.5
Common stock, shares outstanding	136.4			39.5
Post distribution of ownership, shares			78.1
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