Earnings per Share - EPS Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Numerator:
Net income	$ 64.0	$ 55.3	$ 105.7	$ 74.8
Less: Allocation to participating securities	(1.8)	(1.7)	(3.1)	(2.2)
Net income allocated to common stockholders	$ 62.2	$ 53.6	$ 102.6	$ 72.6
Denominator:
Common shares outstanding (in shares)	19,573,871	19,112,216	19,477,540	19,025,604
Dilutive potential common shares outstanding:
Share-based awards (in shares)	438,838	558,323	452,507	544,424
Diluted weighted - average common shares (in shares)	20,012,709	19,670,539	19,930,047	19,570,028