Condensed Consolidated Statements of Stockholders' Equity Statement - 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, beginning of period at Sep. 30, 2019		$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 489.0	$ 0.0	$ (2.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax							(2.3)
Foreign currency translation adjustments								0.4
Impact of initial public offering					(2,112.4)	29.9
Distribution to Post Holdings, Inc.					(11.7)
Reclassification of net investment of Post Holdings, Inc.					524.4	(524.4)
Redemption value adjustment to redeemable noncontrolling interest				(1.1)	(269.2)
Issuance of common stock			0.4		(0.4)
Activity under stock and deferred compensation plans				0.0
Non-cash stock-based compensation expense				1.1
Net earnings available to Class A Common Stockholders	$ 10.2				10.2
Net earnings attributable to Post Holdings, Inc.						5.5
Stockholders' Equity, end of period at Mar. 31, 2020	(1,863.2)	0.0	0.4	0.0	(1,859.1)	0.0	(2.3)	(2.2)
Stockholders' Equity, beginning of period at Dec. 31, 2019		0.0	0.4	0.3	(2,276.9)	0.0	0.2	(2.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax							(2.5)
Foreign currency translation adjustments								0.0
Impact of initial public offering					4.7	0.0
Distribution to Post Holdings, Inc.					(11.7)
Reclassification of net investment of Post Holdings, Inc.					0.0	0.0
Redemption value adjustment to redeemable noncontrolling interest				(1.1)	420.6
Issuance of common stock			0.0		0.0
Activity under stock and deferred compensation plans				0.0
Non-cash stock-based compensation expense				0.8
Net earnings available to Class A Common Stockholders	4.2				4.2
Net earnings attributable to Post Holdings, Inc.						0.0
Stockholders' Equity, end of period at Mar. 31, 2020	(1,863.2)	0.0	0.4	0.0	(1,859.1)	0.0	(2.3)	(2.2)
Stockholders' Equity, beginning of period at Sep. 30, 2020	(2,182.6)	0.0	0.4	0.0	(2,179.0)	0.0	(2.1)	(1.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax							0.3
Foreign currency translation adjustments								0.0
Impact of initial public offering					0.0	0.0
Distribution to Post Holdings, Inc.					(10.7)
Reclassification of net investment of Post Holdings, Inc.					0.0	0.0
Redemption value adjustment to redeemable noncontrolling interest				(1.4)	(250.6)
Issuance of common stock			0.0		0.0
Activity under stock and deferred compensation plans				(0.8)
Non-cash stock-based compensation expense				2.2
Net earnings available to Class A Common Stockholders	8.4				8.4
Net earnings attributable to Post Holdings, Inc.						0.0
Stockholders' Equity, end of period at Mar. 31, 2021	(2,435.2)	0.0	0.4	0.0	(2,431.9)	0.0	(1.8)	(1.9)
Stockholders' Equity, beginning of period at Dec. 31, 2020		0.0	0.4	0.0	(2,496.5)	0.0	(2.0)	(1.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax							0.2
Foreign currency translation adjustments								(0.3)
Impact of initial public offering					0.0	0.0
Distribution to Post Holdings, Inc.					(7.1)
Reclassification of net investment of Post Holdings, Inc.					0.0	0.0
Redemption value adjustment to redeemable noncontrolling interest				(1.2)	71.1
Issuance of common stock			0.0		0.0
Activity under stock and deferred compensation plans				0.1
Non-cash stock-based compensation expense				1.1
Net earnings available to Class A Common Stockholders	0.6				0.6
Net earnings attributable to Post Holdings, Inc.						0.0
Stockholders' Equity, end of period at Mar. 31, 2021	$ (2,435.2)	$ 0.0	$ 0.4	$ 0.0	$ (2,431.9)	$ 0.0	$ (1.8)	$ (1.9)