Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Diluted, by Common Class, Including Two Class Method
Net earnings available to Class A common stockholders, basic	$ 27.6	$ 23.5	$ 0.0
Dilutive securities, effect on basic earnings per share	0.2	0.1
Net earnings available to Class A common stockholders, diluted	$ 27.8	$ 23.6
Weighted-Average Class A common shares outstanding, basic (in shares)	39.5	39.4	0.0
Incremental common shares attributable to dilutive effect of share-based payment arrangements	0.2	0.1
Weighted-Average Class A common shares outstanding, diluted (in shares)	39.7	39.5	0.0
Earnings per Class A common share, basic (in usd per share)	$ 0.70	$ 0.60	$ 0
Earnings per Class A common share, diluted (in usd per share)	$ 0.70	$ 0.60	$ 0
Antidilutive securities excluded from computation of earnings per share, amount	0.2	0.1