Condensed Consolidated Statements of Stockholders' Equity Statement - USD ($) $ in Millions	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Hedging Adjustments, net of tax	Foreign Currency Translation Adjustments	Treasury Stock
Stockholders' Equity, beginning of period at Sep. 30, 2020		$ 0.0	$ 0.4	$ 0.0	$ (2,179.0)	$ (2.1)	$ (1.9)	$ 0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of Spin-off			0.0
Activity under stock and deferred compensation plans				(0.8)
Non-cash stock-based compensation expense				3.4
Redemption value adjustment to redeemable noncontrolling interest				(2.6)	(973.3)
Net earnings available to common stockholders	$ 17.9				17.9
Distribution to Post Holdings, Inc.					(17.5)
Impact of Spin-off					0.0			0.0
Net change in hedges, net of tax						0.4
Foreign currency translation adjustments							0.1
Purchases of treasury stock								0.0
Stockholders' Equity, end of period at Jun. 30, 2021	(3,155.0)	0.0	0.4	0.0	(3,151.9)	(1.7)	(1.8)	0.0
Stockholders' Equity, beginning of period at Mar. 31, 2021		0.0	0.4	0.0	(2,431.9)	(1.8)	(1.9)	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of Spin-off			0.0
Activity under stock and deferred compensation plans				0.0
Non-cash stock-based compensation expense				1.2
Redemption value adjustment to redeemable noncontrolling interest				(1.2)	(722.7)
Net earnings available to common stockholders	9.5				9.5
Distribution to Post Holdings, Inc.					(6.8)
Impact of Spin-off					0.0			0.0
Net change in hedges, net of tax						0.1
Foreign currency translation adjustments							0.1
Purchases of treasury stock								0.0
Stockholders' Equity, end of period at Jun. 30, 2021	(3,155.0)	0.0	0.4	0.0	(3,151.9)	(1.7)	(1.8)	0.0
Stockholders' Equity, beginning of period at Sep. 30, 2021	(3,062.8)	0.0	0.4	0.0	(3,059.7)	(1.6)	(1.9)	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of Spin-off			1.0
Activity under stock and deferred compensation plans				(0.9)
Non-cash stock-based compensation expense				6.8
Redemption value adjustment to redeemable noncontrolling interest				(1.9)	372.4
Net earnings available to common stockholders	48.6				48.6
Distribution to Post Holdings, Inc.					(3.2)
Impact of Spin-off					2,252.6			18.1
Net change in hedges, net of tax						1.6
Foreign currency translation adjustments							(1.4)
Purchases of treasury stock								(20.5)
Stockholders' Equity, end of period at Jun. 30, 2022	(389.6)	0.0	1.4	4.0	(389.3)	0.0	(3.3)	(2.4)
Stockholders' Equity, beginning of period at Mar. 31, 2022		0.0	1.4	0.4	(428.4)	0.0	(2.2)	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of Spin-off			0.0
Activity under stock and deferred compensation plans				0.1
Non-cash stock-based compensation expense				3.5
Redemption value adjustment to redeemable noncontrolling interest				0.0	0.0
Net earnings available to common stockholders	39.1				39.1
Distribution to Post Holdings, Inc.					0.0
Impact of Spin-off					0.0			0.0
Net change in hedges, net of tax						0.0
Foreign currency translation adjustments							(1.1)
Purchases of treasury stock								(2.4)
Stockholders' Equity, end of period at Jun. 30, 2022	$ (389.6)	$ 0.0	$ 1.4	$ 4.0	$ (389.3)	$ 0.0	$ (3.3)	$ (2.4)