Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Earnings Per Share, Diluted, by Common Class, Including Two Class Method
Net earnings available to common stockholders, basic	$ 165.5	$ 82.3	$ 27.6
Dilutive securities, effect on basic earnings per share	0.0	0.0	0.2
Net earnings available to common stockholders, diluted	$ 165.5	$ 82.3	$ 27.8
Weighted-Average common shares outstanding, basic (in shares)	133.0	93.5	39.5
Weighted-Average common shares outstanding, diluted (in shares)	134.1	93.8	39.7
Earnings per common share, basic (in usd per share)	$ 1.24	$ 0.88	$ 0.70
Earnings per common share, diluted (in usd per share)	$ 1.23	$ 0.88	$ 0.70
Restricted Stock Units (RSUs)
Earnings Per Share, Diluted, by Common Class, Including Two Class Method
Antidilutive securities excluded from computation of earnings per share, amount	0.1	0.1	0.0
Incremental common shares attributable to dilutive effect of share-based payment arrangements	0.3	0.2	0.2
BRBR Stock Option
Earnings Per Share, Diluted, by Common Class, Including Two Class Method
Antidilutive securities excluded from computation of earnings per share, amount	0.0	0.0	0.2
Incremental common shares attributable to dilutive effect of share-based payment arrangements	0.1	0.1	0.0
Performance-Based Restricted Stock Units
Earnings Per Share, Diluted, by Common Class, Including Two Class Method
Antidilutive securities excluded from computation of earnings per share, amount	0.1	0.1	0.0
Incremental common shares attributable to dilutive effect of share-based payment arrangements	0.7	0.0	0.0