Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Net investment of Post	Hedging Adjustments, net of tax	Foreign Currency Translation Adjustments
Stockholders' Equity at Sep. 30, 2017		$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 485.4	$ 0.0	$ (1.0)
Preferred stock, shares at Sep. 30, 2017		0
Common stock, shares at Sep. 30, 2017			0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax							0.0
Foreign currency translation adjustments								(0.4)
Impact of initial public offering					0.0	0.0
Distribution declared to Post Holdings, Inc.					0.0
Reclassification of net investment of Post Holdings, Inc.					0.0	0.0
Redemption value adjustment to noncontrolling interest				0.0	0.0
Issuance of common stock			$ 0.0		0.0
Issuance of common stock, shares			0
Activity under deferred compensation plans				0.0
Stock-based compensation expense				0.0
Net earnings available to Class A common stockholders	$ 0.0				0.0
Net earnings attributable to redeemable noncontrolling interest prior to IPO						96.1
Net decrease in net investment of Post Holdings, Inc.						(128.4)
Stockholders' Equity at Sep. 30, 2018	451.7	$ 0.0	$ 0.0	0.0	0.0	453.1	0.0	(1.4)
Preferred stock, shares at Sep. 30, 2018		0
Common stock, shares at Sep. 30, 2018			0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax							0.0
Foreign currency translation adjustments								(1.2)
Impact of initial public offering					0.0	0.0
Distribution declared to Post Holdings, Inc.					0.0
Reclassification of net investment of Post Holdings, Inc.					0.0	0.0
Redemption value adjustment to noncontrolling interest				0.0	0.0
Issuance of common stock			$ 0.0		0.0
Issuance of common stock, shares			0
Activity under deferred compensation plans				0.0
Stock-based compensation expense				0.0
Net earnings available to Class A common stockholders	0.0				0.0
Net earnings attributable to redeemable noncontrolling interest prior to IPO						123.1
Net decrease in net investment of Post Holdings, Inc.						(87.2)
Stockholders' Equity at Sep. 30, 2019	$ 486.4	$ 0.0	$ 0.0	0.0	0.0	489.0	0.0	(2.6)
Preferred stock, shares at Sep. 30, 2019	0	0
Common stock, shares at Sep. 30, 2019			0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax							(2.1)
Foreign currency translation adjustments								0.7
Impact of initial public offering					(2,112.4)	29.9
Distribution declared to Post Holdings, Inc.					(24.8)
Reclassification of net investment of Post Holdings, Inc.					524.4	(524.4)
Redemption value adjustment to noncontrolling interest				(2.6)	(589.3)
Issuance of common stock			$ 0.4		(0.4)
Issuance of common stock, shares			39,400,000
Activity under deferred compensation plans				0.1
Stock-based compensation expense				2.5
Net earnings available to Class A common stockholders	$ 23.5				23.5
Net earnings attributable to redeemable noncontrolling interest prior to IPO	5.5					5.5
Net decrease in net investment of Post Holdings, Inc.						0.0
Stockholders' Equity at Sep. 30, 2020	$ (2,182.6)	$ 0.0	$ 0.4	$ 0.0	$ (2,179.0)	$ 0.0	$ (2.1)	$ (1.9)
Preferred stock, shares at Sep. 30, 2020	0	0
Common stock, shares at Sep. 30, 2020			39,400,000