Stock-Based Compensation - Stock-Settled RSUs (Details) - BRBR Stock-Settled Restricted Stock Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Share-based Payment Award [Line Items]
RSUs nonvested, beginning balance	579,969
RSUs granted in period	207,224
RSUs vested in period	(302,887)
RSUs forfeited in period	(19,075)
RSUs nonvested, ending balance	465,231	579,969
RSUs nonvested, weighted average grant date fair value	$ 24.06	$ 21.23	$ 19.85
RSUs granted in period, weighted average grant date fair value	26.10
RSUs vested in period, weighted average grant date fair value	20.09
RSUs forfeited in period, weighted average grant date fair value	$ 23.13
RSUs vested in period, fair value	$ 7.8	$ 5.2	$ 3.0