Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions			12 Months Ended
	Nov. 25, 2022	Aug. 11, 2022	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021	Mar. 10, 2022	Mar. 09, 2022	Sep. 30, 2020
Class of Stock [Line Items]
Post distribution of ownership, shares	4,600,000	14,800,000				78,100,000
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding			131,200,000	135,300,000	39,500,000			39,400,000
Purchases of treasury stock, shares			(4,200,000)	(1,900,000)	0
Post Holdings, Inc. | BellRing Brands, LLC unit
Class of Stock [Line Items]
Common units, outstanding							97,500,000
Common Class A
Class of Stock [Line Items]
Common stock, par value per share					$ 0.01
Common Class A | Common Stock
Class of Stock [Line Items]
Purchases of treasury stock, shares				800,000
Treasury Stock Acquired, Average Cost Per Share				$ 23.34
Payments for Repurchase of Common Stock				$ 18.1
Common Stock
Class of Stock [Line Items]
Common stock, shares authorized			500,000,000
Common stock, shares issued			136,553,891	136,362,928
Common stock, par value per share			$ 0.01	$ 0.01
Common stock, shares outstanding			131,245,350	135,295,583
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	900,000	800,000	4,200,000	1,100,000
Treasury Stock Acquired, Average Cost Per Share			$ 29.56	$ 23.17
Payments for Repurchase of Common Stock			$ 126.3	$ 24.7
Sales and Excise Tax Payable, Current			$ 0.8
BellRing Common Stock | Post Holdings, Inc.
Class of Stock [Line Items]
Common units, outstanding						19,400,000