Earnings per Share - EPS Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerator:
Net income	$ 34.2	$ 36.6	$ 109.0	$ 92.2
Less: Allocation to participating securities	(1.1)	(0.9)	(3.2)	(2.1)
Net income allocated to common stockholders	$ 33.1	$ 35.7	$ 105.8	$ 90.1
Denominator:
Common shares outstanding (in shares)	19,255,452	18,807,104	19,102,244	18,809,823
Dilutive potential common shares outstanding:
Share-based awards (in shares)	599,774	296,062	567,153	312,858
Diluted weighted - average common shares (in shares)	19,855,226	19,103,166	19,669,397	19,122,681