Basis of Presentation (Details) - USD ($)		3 Months Ended		6 Months Ended
	Oct. 21, 2019	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Offering price per share of IPO	$ 14.00
Proceeds from IPO	$ 524,400,000
Net earnings attributable to redeemable noncontrolling interest		$ 14,400,000	$ 31,000,000.0	$ 40,200,000	$ 56,100,000
Net earnings attributable to redeemable noncontrolling interest prior to IPO		0		5,500,000
Net earnings attributable to redeemable noncontrolling interest subsequent to IPO		$ 14,400,000		$ 34,700,000
Common Class A
Common stock, shares issued	39,400,000	39,400,000		39,400,000
Common stock, par value per share	$ 0.01
Common stock, shares issued per underwriters' option	5,100,000
Voting power of common stock	33.00%
Common Class B
Common stock, shares issued	1	1		1
Common stock, par value per share	$ 0.01
Voting power of common stock	67.00%
BellRing Brands, LLC unit | BellRing Brands, Inc.
Common unit, issued	39,400,000
Noncontrolling interest, ownership percentage by parent	28.80%	28.80%		28.80%
BellRing Brands, LLC unit | Post Holdings, Inc.
Common unit, issued	97,500,000
Noncontrolling interest, ownership percentage by noncontrolling owners	71.20%	71.20%		71.20%		100.00%