Summary of Significant Accounting Policies - Basis (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Oct. 21, 2019
Basis of Consolidation [Line Items]
Net earnings attributable to redeemable noncontrolling interest	$ 76.6	$ 123.1	$ 96.1
Net earnings attributable to redeemable noncontrolling interest prior to IPO	5.5
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO	$ 71.1
Post Holdings, Inc. | BellRing Brands, LLC unit
Basis of Consolidation [Line Items]
Noncontrolling interest, ownership percentage by noncontrolling owners	71.20%	100.00%		71.20%
BellRing Brands, Inc. | BellRing Brands, LLC unit
Basis of Consolidation [Line Items]
Noncontrolling interest, ownership percentage by parent	28.80%			28.80%