Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure
Effective income tax rate	7.10%	8.40%	24.20%
Undistributed earnings of foreign subsidiaries	$ 1.0	$ 2.3
Income (loss) from continuing operations before income taxes, foreign	$ (1.9)	$ (0.8)	$ 1.0
BellRing Brands, LLC unit | BellRing Brands, Inc.
Income Tax Disclosure
Noncontrolling interest, ownership percentage by parent	28.80%	28.80%