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	Treasury Stock
Stockholders' Equity at Sep. 30, 2019		$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 489.0	$ 0.0	$ (2.6)	$ 0.0
Preferred stock, shares at Sep. 30, 2019		0
Common stock, shares at Sep. 30, 2019			0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of Spin-off, Amount	$ 0.0
Net change in hedges, net of tax							(2.1)
Foreign currency translation adjustments								0.7
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)
Impact of Spin-off					0.0				0.0
Initial public offering issuance of common stock			$ 0.4		(0.4)
Initial public offering					(2,112.4)	29.9
Issuance of common stock, shares			39,400,000
Purchases of treasury stock, shares			0
Purchases of treasury stock									0.0
Shares Issued, Shares, Share-based Payment Arrangement, before Forfeiture			0
Impact of Spin-off, Shares			0
Activity under deferred compensation plans				0.1
Stock-based compensation expense				2.5
Net earnings available to common stockholders	23.5				23.5
Net earnings attributable to Post Holdings, Inc.						5.5
Stockholders' Equity at Sep. 30, 2020	(2,182.6)	$ 0.0	$ 0.4	0.0	(2,179.0)	0.0	(2.1)	(1.9)	0.0
Preferred stock, shares at Sep. 30, 2020		0
Common stock, shares at Sep. 30, 2020			39,400,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of Spin-off, Amount	0.0
Net change in hedges, net of tax							0.5
Foreign currency translation adjustments								0.0
Distribution declared to Post Holdings, Inc.					(24.6)
Reclassification of net investment of Post Holdings, Inc.					0.0	0.0
Redemption value adjustment to noncontrolling interest				(3.8)	(883.7)
Impact of Spin-off					0.0				0.0
Initial public offering issuance of common stock			$ 0.0		0.0
Initial public offering					0.0	0.0
Issuance of common stock, shares			0
Purchases of treasury stock, shares			0
Purchases of treasury stock									0.0
Shares Issued, Shares, Share-based Payment Arrangement, before Forfeiture			100,000
Impact of Spin-off, Shares			0
Activity under deferred compensation plans				(0.8)
Stock-based compensation expense				4.6
Net earnings available to common stockholders	27.6				27.6
Net earnings attributable to Post Holdings, Inc.						0.0
Stockholders' Equity at Sep. 30, 2021	$ (3,062.8)	$ 0.0	$ 0.4	0.0	(3,059.7)	0.0	(1.6)	(1.9)	0.0
Preferred stock, shares at Sep. 30, 2021	0	0
Common stock, shares at Sep. 30, 2021			39,500,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Impact of Spin-off, Amount			$ 1.0
Net change in hedges, net of tax							1.6
Foreign currency translation adjustments								(2.4)
Distribution declared to Post Holdings, Inc.					(3.2)
Reclassification of net investment of Post Holdings, Inc.					0.0	0.0
Redemption value adjustment to noncontrolling interest				(1.9)	372.4
Impact of Spin-off					2,252.6				18.1
Initial public offering issuance of common stock			$ 0.0		0.0
Initial public offering					0.0	0.0
Issuance of common stock, shares			0
Purchases of treasury stock, shares			(1,900,000)
Purchases of treasury stock									(42.8)
Shares Issued, Shares, Share-based Payment Arrangement, before Forfeiture			200,000
Impact of Spin-off, Shares			97,500,000
Activity under deferred compensation plans				(0.9)
Stock-based compensation expense				9.8
Net earnings available to common stockholders	$ 82.3				82.3
Net earnings attributable to Post Holdings, Inc.						0.0
Stockholders' Equity at Sep. 30, 2022	$ (376.2)	$ 0.0	$ 1.4	$ 7.0	$ (355.6)	$ 0.0	$ 0.0	$ (4.3)	$ (24.7)
Preferred stock, shares at Sep. 30, 2022	0	0
Common stock, shares at Sep. 30, 2022			135,300,000