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Background (Details) - USD ($)
$ / shares in Units, $ in Millions
Oct. 21, 2019
Sep. 30, 2020
Sep. 30, 2019
Common Class A      
Common stock, par value per share $ 0.01 $ 0.01 $ 0.01
Issuance of common stock, shares 39,400,000    
Common stock, shares issued per underwriters' option 5,100,000    
Proceeds from IPO $ 524.4    
Voting power of common stock 33.00%    
Common Class B      
Common stock, par value per share $ 0.01 $ 0.01 $ 0.01
Issuance of common stock, shares 1    
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%  
BellRing Brands, LLC unit | Post Holdings, Inc.      
Common unit, issued 97,500,000    
Noncontrolling interest, ownership percentage by noncontrolling owners 71.20% 71.20% 100.00%