Income Taxes - Deferreds (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure
Deferred tax liabilities, investment in noncontrolled affiliates	$ 12.6
Deferred tax asset, investment in BellRing LLC	3.0
Deferred tax liability, investment in BellRing LLC's subsidiaries	15.6
Assets | Noncurrent
Income Tax Disclosure
Deferred tax assets, employee compensation	0.0	$ 2.1
Deferred tax assets, share-based compensation cost	0.0	0.8
Deferred tax assets, inventory	0.0	3.1
Deferred tax assets, accrued liabilities	2.6	2.2
Deferred tax assets (liabilities), intangible assets	1.0	0.0
Deferred tax liabilities, property, plant and equipment	0.0	0.0
Deferred tax liabilities, investment in noncontrolled affiliates	0.0	0.0
Deferred tax assets, net	3.6	8.2
Liability | Noncurrent
Income Tax Disclosure
Deferred tax assets, employee compensation	0.0	0.0
Deferred tax assets, share-based compensation cost	0.0	0.0
Deferred tax assets, inventory	0.0	0.0
Deferred tax assets, accrued liabilities	0.0	0.0
Deferred tax assets (liabilities), intangible assets	0.0	(21.9)
Deferred tax liabilities, property, plant and equipment	0.0	(0.4)
Deferred tax liabilities, investment in noncontrolled affiliates	(12.6)	0.0
Deferred tax liabilities, net	(12.6)	(22.3)
Net Asset (Liability) | Noncurrent
Income Tax Disclosure
Deferred tax assets, employee compensation	0.0	2.1
Deferred tax assets, share-based compensation cost	0.0	0.8
Deferred tax assets, inventory	0.0	3.1
Deferred tax assets, accrued liabilities	2.6	2.2
Deferred tax assets (liabilities), intangible assets	1.0	(21.9)
Deferred tax liabilities, property, plant and equipment	0.0	(0.4)
Deferred tax liabilities, investment in noncontrolled affiliates	(12.6)	0.0
Deferred tax liabilities, net	$ (9.0)	$ (14.1)