Earnings per Share - EPS Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Numerator:
Net income	$ 53.1	$ 41.7	$ 122.2	$ 118.3
Less: Allocation to participating securities	(1.9)	(1.5)	(4.3)	(4.0)
Net income allocated to common stockholders, basic	51.2	40.2	117.9	114.3
Net income allocated to common stockholders, diluted	$ 51.2	$ 40.2	$ 117.9	$ 114.3
Denominator:
Common shares outstanding (in shares)	30,473,856	29,895,041	30,352,824	29,775,078
Dilutive potential common shares outstanding:
Share-based awards (in shares)	1,025,087	1,036,751	1,020,915	1,055,792
Diluted weighted - average common shares (in shares)	31,498,943	30,931,792	31,373,739	30,830,870