Basis of Presentation (Details) - USD ($)		3 Months Ended
	Oct. 21, 2019	Dec. 31, 2019	Dec. 31, 2018	Oct. 01, 2019
Offering Price Per Share of IPO	$ 14.00
Proceeds from Issuance IPO	$ 524,400,000
Net earnings attributable to redeemable noncontrolling interest		$ 25,800,000	$ 25,100,000
Net earnings attributable to redeemable noncontrolling interest prior to IPO		5,500,000
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO		$ 20,300,000
Common Class A [Member]
Common stock, shares issued	39,400,000	39,400,000
Common stock, par value per share	$ 0.01
Common stock, shares issued per underwriters' option	5,100,000
Common Class B [Member]
Common stock, shares issued	1	1
Common stock, par value per share	$ 0.01
BellRing Brands, LLC unit [Member] | BellRing Brands, Inc. [Member]
Common Unit, Issued	39,400,000
Noncontrolling Interest, Ownership Percentage by Parent	28.80%	28.80%
BellRing Brands, LLC unit [Member] | Post Holdings, Inc. [Member]
Common Unit, Issued	97,500,000
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners	71.20%	71.20%		100.00%