Stock-Based Compensation - Narrative (Details) - BellRing 2019 Long-Term Incentive Plan - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Share-based Payment Award [Line Items]
Maximum award vesting period	10 years
Share-based payment arrangement, expense	$ 22.1	$ 21.0	$ 14.2
Share-based payment arrangement, expense, tax benefit	1.6	$ 2.0	$ 1.6
Share-based payment arrangement, nonvested award, cost not yet recognized, amount	$ 24.7
Share-based payment arrangement, nonvested award, cost not yet recognized, period for recognition	10 months 24 days
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	6.2
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Additional Shares Authorized	6.0