Stock-Based Compensation - Stock-Settled RSUs (Details) - BRBR Stock-Settled Restricted Stock Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Share-based Payment Award [Line Items]
RSUs nonvested, beginning balance	433,260
RSUs granted in period	132,824
RSUs vested in period	(198,417)
RSUs forfeited in period	(17,972)
RSUs nonvested, ending balance	349,695	433,260
RSUs nonvested, weighted average grant date fair value	$ 42.48	$ 36.63	$ 24.06
RSUs granted in period, weighted average grant date fair value	69.67
RSUs vested in period, weighted average grant date fair value	31.04
RSUs forfeited in period, weighted average grant date fair value	$ 39.77
RSUs vested in period, fair value	$ 14.4	$ 11.8	$ 7.8