Redeemable Noncontrolling Interest (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Sep. 30, 2021
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	$ 2,997.3
Net earnings available to Class A Common Stockholders	8.2	$ 7.8
Redeemable noncontrolling interest, end of period	2,780.9
Level 1
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	2,997.3
Redeemable noncontrolling interest, end of period	2,780.9
BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Net earnings available to Class A Common Stockholders	8.2	7.8
Redemption value adjustment to redeemable noncontrolling interest	(247.6)	321.9
Changes from net earnings available to Class A Common Stockholders and transfers to NCI	$ (239.4)	329.7
BellRing Brands, LLC unit | Post Holdings, Inc.
Redeemable Noncontrolling Interest [Line Items]
Common units, outstanding	97.5		97.5
Noncontrolling interest, ownership percentage by noncontrolling owners	71.50%		71.20%
BellRing Brands, LLC unit | BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by parent	28.50%		28.80%
Redeemable noncontrolling interest
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	$ 2,997.3	2,021.6
Net earnings attributable to NCI	31.1	25.1
Net change in hedges, net of tax	0.4	0.4
Foreign currency translation adjustments	(0.3)	0.6
Redemption value adjustment to redeemable noncontrolling interest	(247.6)	321.9
Redeemable noncontrolling interest, end of period	$ 2,780.9	$ 2,369.6