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, 2018		$ 0.0	$ 0.0	$ 0.0		$ 453.1	$ 0.0	$ (1.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net change in hedges, net of tax							0.0
Foreign currency translation adjustments								(0.7)
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
Stock Issued During Period, Value, New Issues			0.0		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	56.1					56.1
Net (decrease) increase in net investment of Post Holdings, Inc.						(2.5)
Stockholders' Equity, end of period at Mar. 31, 2019	504.6	0.0	0.0	0.0	0.0	506.7	0.0	(2.1)
Stockholders' Equity, beginning of period at Dec. 31, 2018			0.0	0.0	0.0	474.2	0.0	(1.7)
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
Stock Issued During Period, Value, New Issues			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	31.0					31.0
Net (decrease) increase in net investment of Post Holdings, Inc.						1.5
Stockholders' Equity, end of period at Mar. 31, 2019	504.6	0.0	0.0	0.0	0.0	506.7	0.0	(2.1)
Stockholders' Equity, beginning of period at Sep. 30, 2019	486.4	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 declared to Post Holdings, Inc.					(11.7)
Reclassification of net investment of Post Holdings, Inc.					524.4	(524.4)
Redemption value adjustment to noncontrolling interest				(1.1)	(269.2)
Stock Issued During Period, Value, New Issues			0.4		(0.4)
Stock-based compensation expense				1.1
Net earnings available to Class A common stockholders	10.2				10.2
Net earnings attributable to redeemable noncontrolling interest prior to IPO	5.5					5.5
Net earnings attributable to redeemable noncontrolling interest	40.2
Net (decrease) increase in net investment of 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 Dec. 31, 2019			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 declared to Post Holdings, Inc.					(11.7)
Reclassification of net investment of Post Holdings, Inc.					0.0	0.0
Redemption value adjustment to noncontrolling interest				(1.1)	420.6
Stock Issued During Period, Value, New Issues			0.0		0.0
Stock-based compensation expense				0.8
Net earnings available to Class A common stockholders	4.2				4.2
Net earnings attributable to redeemable noncontrolling interest prior to IPO	0.0					0.0
Net earnings attributable to redeemable noncontrolling interest	14.4
Net (decrease) increase in net investment of 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)