Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Aug. 11, 2022	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020	May 23, 2022	Mar. 10, 2022	Mar. 09, 2022	Sep. 30, 2019
Class of Stock [Line Items]
Share Repurchase Authorization					$ 50.0
Payments for Repurchase of Common Stock		$ 42.8	$ 0.0	$ 0.0
Post distribution of ownership, shares	14,800,000					78,100,000
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding		135,300,000	39,500,000	39,400,000				0
Purchases of treasury stock, shares		(1,900,000)	0	0
Old BellRing Stockholders
Class of Stock [Line Items]
Common stock, shares issued			39,500,000			38,900,000
Common stock, shares outstanding			39,500,000
Post Holdings, Inc. | BellRing Brands, LLC unit
Class of Stock [Line Items]
Common units, outstanding			97,500,000	97,500,000		19,400,000	97,500,000
Common Class A
Class of Stock [Line Items]
Common stock, shares authorized		0	500,000,000
Common stock, shares issued		0	39,510,430
Common stock, par value per share		$ 0.01	$ 0.01
Common stock, shares outstanding		0	39,510,430
Common Class A | Common Stock
Class of Stock [Line Items]
Purchases of treasury stock, shares	800,000	800,000
Treasury Stock Acquired, Average Cost Per Share		$ 23.36
Payments for Repurchase of Common Stock		$ 18.1
Common Class B
Class of Stock [Line Items]
Common stock, shares authorized		0	1
Common stock, shares issued		0	1
Common stock, par value per share			$ 0.01
Common stock, shares outstanding		0	1
Common Stock
Class of Stock [Line Items]
Common stock, shares authorized		500,000,000	0
Common stock, shares issued		136,362,928	0
Common stock, par value per share		$ 0.01	$ 0.01
Common stock, shares outstanding		135,295,583	0
BellRing Common Stock
Class of Stock [Line Items]
Common stock, par value per share		$ 0.01
BellRing Common Stock | Common Stock
Class of Stock [Line Items]
Purchases of treasury stock, shares		1,100,000
Treasury Stock Acquired, Average Cost Per Share		$ 23.18
Payments for Repurchase of Common Stock		$ 24.7