Income Taxes - Deferreds (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020
Income Tax Disclosure
Deferred tax liabilities, investment in noncontrolled affiliates	$ 11.2	$ 12.6
Deferred tax asset, investment in BellRing LLC	5.4	3.0
Deferred tax liability, investment in BellRing LLC's subsidiaries	16.6	15.6
Assets | Noncurrent
Income Tax Disclosure
Deferred tax assets, share-based compensation cost	0.1	0.0
Deferred tax assets, accrued liabilities	2.5	2.6
Deferred tax assets (liabilities), intangible assets	1.0	1.0
Deferred tax liabilities, investment in noncontrolled affiliates	0.0	0.0
Deferred tax assets, net	3.6	3.6
Liability | Noncurrent
Income Tax Disclosure
Deferred tax assets, share-based compensation cost	0.0	0.0
Deferred tax assets, accrued liabilities	0.0	0.0
Deferred tax assets (liabilities), intangible assets	0.0	0.0
Deferred tax liabilities, investment in noncontrolled affiliates	(11.2)	(12.6)
Deferred tax liabilities, net	(11.2)	(12.6)
Net Asset (Liability) | Noncurrent
Income Tax Disclosure
Deferred tax assets, share-based compensation cost	0.1	0.0
Deferred tax assets, accrued liabilities	2.5	2.6
Deferred tax assets (liabilities), intangible assets	1.0	1.0
Deferred tax liabilities, investment in noncontrolled affiliates	(11.2)	12.6
Deferred tax liabilities, net	$ (7.6)	$ (9.0)