Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Hedging Adjustments, net of tax	Foreign Currency Translation Adjustments	Treasury Stock, Common
Stockholders' Equity at Sep. 30, 2020		$ 0.0	$ 0.4	$ 0.0	$ (2,179.0)	$ (2.1)	$ (1.9)	$ 0.0
Preferred stock, shares at Sep. 30, 2020		0.0
Common stock, shares at Sep. 30, 2020			39.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax						0.5
Foreign currency translation adjustments							0.0
Distributions to Post Holdings, Inc.					(24.6)
Redemption value adjustment to noncontrolling interest				(3.8)	(883.7)
Impact of Spin-off			$ 0.0		0.0			0.0
Purchases of treasury stock, shares			0.0
Purchases of treasury stock								0.0
Shares Issued, Shares, Share-based Payment Arrangement, before Forfeiture			0.1
Impact of Spin-off, Shares			0.0
Activity under deferred compensation plans				(0.8)
Stock-based compensation expense				4.6
Net earnings available to common stockholders	$ 27.6				27.6
Stockholders' Equity at Sep. 30, 2021	(3,062.8)	$ 0.0	$ 0.4	0.0	(3,059.7)	(1.6)	(1.9)	0.0
Preferred stock, shares at Sep. 30, 2021		0.0
Common stock, shares at Sep. 30, 2021			39.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax						1.6
Foreign currency translation adjustments							(2.4)
Distributions to Post Holdings, Inc.					(3.2)
Redemption value adjustment to noncontrolling interest				(1.9)	372.4
Impact of Spin-off			$ 1.0		2,252.6			18.1
Purchases of treasury stock, shares			(1.9)
Purchases of treasury stock								(42.8)
Shares Issued, Shares, Share-based Payment Arrangement, before Forfeiture			0.2
Impact of Spin-off, Shares			97.5
Activity under deferred compensation plans				(0.9)
Stock-based compensation expense				9.8
Net earnings available to common stockholders	82.3				82.3
Stockholders' Equity at Sep. 30, 2022	(376.2)	$ 0.0	$ 1.4	7.0	(355.6)	0.0	(4.3)	(24.7)
Preferred stock, shares at Sep. 30, 2022		0.0
Common stock, shares at Sep. 30, 2022			135.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax						0.0
Foreign currency translation adjustments							1.2
Distributions to Post Holdings, Inc.					0.0
Redemption value adjustment to noncontrolling interest				0.0	0.0
Impact of Spin-off			$ 0.0		0.0			0.0
Purchases of treasury stock, shares			(4.2)
Purchases of treasury stock								(126.3)
Shares Issued, Shares, Share-based Payment Arrangement, before Forfeiture			0.1
Impact of Spin-off, Shares			0.0
Activity under deferred compensation plans				(1.9)
Stock-based compensation expense				14.2
Net earnings available to common stockholders	165.5				165.5
Stockholders' Equity at Sep. 30, 2023	$ (323.5)	$ 0.0	$ 1.4	$ 19.3	$ (190.1)	$ 0.0	$ (3.1)	$ (151.0)
Preferred stock, shares at Sep. 30, 2023		0.0
Common stock, shares at Sep. 30, 2023			131.2