Earnings per Share - EPS Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Numerator:
Net income	$ 41.7	$ 64.0	$ 118.3	$ 105.7
Less: Allocation to participating securities	(1.5)	(1.8)	(4.0)	(3.1)
Net income allocated to common stockholders	$ 40.2	$ 62.2	$ 114.3	$ 102.6
Denominator:
Common shares outstanding (in shares)	19,930,027	19,573,871	19,850,052	19,477,540
Dilutive potential common shares outstanding:
Share-based awards (in shares)	691,167	438,838	703,861	452,507
Diluted weighted - average common shares (in shares)	20,621,194	20,012,709	20,553,913	19,930,047