Summary of Significant Accounting Policies - Taxes (Details)	Sep. 30, 2021	Sep. 30, 2020
BellRing Brands, Inc. \| BellRing Brands, LLC unit
Taxes [Line Items]
Noncontrolling interest, ownership percentage by parent	28.80%	28.80%

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

No definition available.

Name:	us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant