Stock-Based Compensation - Narrative (Details) - BellRing 2019 Long-Term Incentive Plan - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Share-based Payment Award [Line Items]
Maximum award vesting period	10 years
Share-based payment arrangement, expense	$ 14.2	$ 9.8	$ 4.6
Share-based payment arrangement, expense, tax benefit	1.6	$ 1.2	$ 0.3
Share-based payment arrangement, nonvested award, cost not yet recognized, amount	$ 19.5
Share-based payment arrangement, nonvested award, cost not yet recognized, period for recognition	1 year 3 months 18 days
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	1.4