Subsequent Event (Notes)	12 Months Ended
Sep. 30, 2021
Subsequent Event [Abstract]
Subsequent Event	SUBSEQUENT EVENTIn August 2021, Post announced its plan to distribute a significant portion of its ownership interest in the Company to its shareholders and in October 2021, the Company and Post announced the signing of a Transaction Agreement and Plan of Merger (the “Transaction Agreement”) between them, BellRing Distribution, LLC, a newly-formed wholly-owned subsidiary of Post (“New BellRing”), and BellRing Merger Sub Corporation, a wholly-owned subsidiary of New BellRing (“Merger Sub”) related to Post’s distribution plan. Pursuant to the Transaction Agreement, Post will contribute its one share of Class B Common Stock, all of its BellRing LLC units and cash to New BellRing in exchange for all of the then-outstanding equity of New BellRing and New BellRing indebtedness (the “Separation”). New BellRing will convert into a Delaware corporation, and Post will then distribute at least 80.1% of its shares of New BellRing common stock to Post shareholders in a pro-rata distribution, an exchange offer or a combination of both, depending on market conditions. Upon completion of the distribution of New BellRing common stock to Post shareholders (the “Distribution”), Merger Sub will merge with and into BellRing Inc. (the “Merger”), with BellRing Inc. as the surviving corporation and a wholly-owned subsidiary of New BellRing. Pursuant to the Merger, each outstanding share of Class A Common Stock will be converted into one share of New BellRing common stock plus a to-be-determined amount of cash per share. The exact cash consideration will be determined in accordance with the Transaction Agreement based upon several factors, including the amount of New BellRing indebtedness to be issued. Immediately following the Distribution and Merger, it is expected that Post will own no more than 14.2% of the New BellRing common stock and the Post shareholders will own at least 57.0% of the New BellRing common stock. Legacy Class A common stockholders will own approximately 28.8% of the New BellRing common stock, maintaining their current effective ownership interest in the Company. The Transaction Agreement also contemplates that Post and New BellRing will enter into certain customary ancillary agreements in connection with the consummation of the Merger. Completion of the transactions is anticipated to occur in the first calendar quarter of 2022, subject to certain customary closing conditions, although there can be no assurance that the transactions will occur within the expected timeframe or at all.

Subsequent Event (Notes)

12 Months Ended

Sep. 30, 2021

SUBSEQUENT EVENTIn August 2021, Post announced its plan to distribute a significant portion of its ownership interest in the Company to its shareholders and in October 2021, the Company and Post announced the signing of a Transaction Agreement and Plan of Merger (the “Transaction Agreement”) between them, BellRing Distribution, LLC, a newly-formed wholly-owned subsidiary of Post (“New BellRing”), and BellRing Merger Sub Corporation, a wholly-owned subsidiary of New BellRing (“Merger Sub”) related to Post’s distribution plan. Pursuant to the Transaction Agreement, Post will contribute its one share of Class B Common Stock, all of its BellRing LLC units and cash to New BellRing in exchange for all of the then-outstanding equity of New BellRing and New BellRing indebtedness (the “Separation”). New BellRing will convert into a Delaware corporation, and Post will then distribute at least 80.1% of its shares of New BellRing common stock to Post shareholders in a pro-rata distribution, an exchange offer or a combination of both, depending on market conditions. Upon completion of the distribution of New BellRing common stock to Post shareholders (the “Distribution”), Merger Sub will merge with and into BellRing Inc. (the “Merger”), with BellRing Inc. as the surviving corporation and a wholly-owned subsidiary of New BellRing. Pursuant to the Merger, each outstanding share of Class A Common Stock will be converted into one share of New BellRing common stock plus a to-be-determined amount of cash per share. The exact cash consideration will be determined in accordance with the Transaction Agreement based upon several factors, including the amount of New BellRing indebtedness to be issued. Immediately following the Distribution and Merger, it is expected that Post will own no more than 14.2% of the New BellRing common stock and the Post shareholders will own at least 57.0% of the New BellRing common stock. Legacy Class A common stockholders will own approximately 28.8% of the New BellRing common stock, maintaining their current effective ownership interest in the Company. The Transaction Agreement also contemplates that Post and New BellRing will enter into certain customary ancillary agreements in connection with the consummation of the Merger. Completion of the transactions is anticipated to occur in the first calendar quarter of 2022, subject to certain customary closing conditions, although there can be no assurance that the transactions will occur within the expected timeframe or at all.