Earnings per Share - EPS Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Numerator:
Net income	$ 69.5	$ 49.1	$ 187.8	$ 154.8
Less: Allocation to participating securities	(2.4)	(1.5)	(6.3)	(4.5)
Net income allocated to common stockholders	$ 67.1	$ 47.6	$ 181.5	$ 150.3
Denominator:
Common shares outstanding (in shares)	20,040,167	19,634,450	19,913,424	19,529,843
Dilutive potential common shares outstanding:
Share-based awards (in shares)	614,133	475,542	664,891	455,055
Diluted weighted - average common shares (in shares)	20,654,300	20,109,992	20,578,315	19,984,898