Stock-Based Compensation - Performance-based RSUs (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Minimum
Share-based Payment Award [Line Items]
Total Award Range	0.00%
Maximum
Share-based Payment Award [Line Items]
Total Award Range	260.00%
BRBR Performance-Based Restricted Stock Units
Share-based Payment Award [Line Items]
RSUs nonvested, beginning balance	773,194
RSUs granted in period	85,818
Adjustment for performance achievement (a)	600,351
RSUs vested in period	(975,570)
RSUs forfeited in period	0
RSUs nonvested, ending balance	483,793	773,194
RSUs nonvested, weighted average grant date fair value	$ 77.47	$ 53.79
RSUs granted in period, weighted average grant date fair value	129.89
RSUs vested in period, weighted average grant date fair value	41.44
RSUs forfeited in period, weighted average grant date fair value	$ 0
Fair Value Assumptions, expected term	3 years	3 years	3 years
Fair Value Assumptions, expected volatility rate	36.50%	39.40%	46.80%
Fair Value Assumptions, risk free interest rate	4.10%	4.70%	4.10%
Fair Value Assumptions, weighted average grant date fair value	$ 129.89	$ 81.42	$ 45.26
Stock options, expected dividend rate	0.00%	0.00%	0.00%
RSUs vested in period, fair value	$ 75.6