Stock-Based Compensation - Stock Options (Details) - BRBR Stock Options - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Share-based Payment Award [Line Items]
Stock options outstanding, beginning balance	258,987
Stock options granted in period	0
Stock options exercised in period	0
Stock options forfeited in period	0
Stock options expired in period	0
Stock options outstanding, ending balance	258,987	258,987
Stock options vested and expected to vest	258,987
Stock options exercisable	258,987
Stock options outstanding, weighted average exercise price	$ 17.74	$ 17.74
Stock options granted in period, weighted average exercise price	0
Stock options exercised in period, weighted average exercise price	0
Stock options forfeited in period, weighted average exercise price	0
Stock options expired in period, weighted average exercise price	0
Stock options vested and expected to vest, weighted average exercise price	17.74
Stock options exercisable in period, weighted average exercise price	$ 17.74
Stock options outstanding, weighted average remaining contractual term	5 years 9 months 18 days
Stock options vested and expected to vest, weighted average remaining contractual term	5 years 9 months 18 days
Stock options exercisable, weighted average remaining contractual term	5 years 9 months 18 days
Stock options outstanding, intrinsic value	$ 11.1
Stock options vested and expected to vest, intrinsic value	11.1
Stock options exercisable, intrinsic value	$ 11.1
Stock options, exercises in period, intrinsic value			$ 0.1
Proceeds from exercises of stock awards			$ 0.5