Redeemable Noncontrolling Interest (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Oct. 21, 2019	Sep. 30, 2019
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period			$ 0.0
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO	$ 10.9		45.6
Net earnings available to Class A common stockholders	3.3	$ 0.0	13.5	$ 0.0
Redeemable noncontrolling interest, end of period	1,943.6		1,943.6
BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Net earnings available to Class A common stockholders	3.3		13.5
Impact of initial public offering	0.0		1,364.6
Redemption value adjustment to noncontrolling interest	270.1		540.4
Change from net earnings available to class A common stockholders and effects of changes, net	$ 273.4		$ 1,918.5
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	71.20%		71.20%		71.20%	100.00%
BellRing Brands, LLC unit | BellRing Brands, Inc.
Redeemable Noncontrolling Interest [Line Items]
Noncontrolling interest, ownership percentage by parent	28.80%		28.80%		28.80%
Redeemable noncontrolling interest
Redeemable Noncontrolling Interest [Line Items]
Redeemable noncontrolling interest, beginning of period	$ 1,661.9		$ 0.0
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO	10.9		45.6
Net change in hedges, net of tax	0.4		(7.1)
Foreign currency translation adjustments	0.3		0.1
Impact of initial public offering	0.0		1,364.6
Redemption value adjustment to noncontrolling interest	270.1		540.4
Redeemable noncontrolling interest, end of period	$ 1,943.6		$ 1,943.6