Earnings per Share - EPS Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Numerator:
Net income	$ 49.1	$ 34.2	$ 154.8	$ 109.0
Less: Allocation to participating securities	(1.5)	(1.1)	(4.5)	(3.2)
Net income allocated to common stockholders	$ 47.6	$ 33.1	$ 150.3	$ 105.8
Denominator:
Common shares outstanding (in shares)	19,634,450	19,255,452	19,529,843	19,102,244
Dilutive potential common shares outstanding:
Share-based awards (in shares)	475,542	599,774	455,055	567,153
Diluted weighted - average common shares (in shares)	20,109,992	19,855,226	19,984,898	19,669,397