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]
Activity under stock and deferred compensation plans				(0.9)
Non-cash stock-based compensation expense				1.1
Redemption value adjustment to redeemable noncontrolling interest				(0.2)	(321.7)
Net earnings available to Class A Common Stockholders	$ 7.8				7.8
Distribution to Post Holdings, Inc.					(3.6)
Net change in hedges, net of tax						0.1
Foreign currency translation adjustments							0.3
Purchases of treasury stock								0.0
Stockholders' Equity, end of period at Dec. 31, 2020	(2,499.7)	0.0	0.4	0.0	(2,496.5)	(2.0)	(1.6)	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]
Activity under stock and deferred compensation plans				(1.0)
Non-cash stock-based compensation expense				1.5
Redemption value adjustment to redeemable noncontrolling interest				(0.5)	248.1
Net earnings available to Class A Common Stockholders	8.2				8.2
Distribution to Post Holdings, Inc.					(3.2)
Net change in hedges, net of tax						0.1
Foreign currency translation adjustments							(0.1)
Purchases of treasury stock								(18.1)
Stockholders' Equity, end of period at Dec. 31, 2021	$ (2,827.8)	$ 0.0	$ 0.4	$ 0.0	$ (2,806.6)	$ (1.5)	$ (2.0)	$ (18.1)