Earnings per Share - EPS Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income	$ 116.3	$ 169.6	$ 85.1
Less: Allocation to participating securities	(3.5)	(4.0)	(1.5)
Net income allocated to common stockholders	$ 112.8	$ 165.6	$ 83.6
Denominator:
Common shares outstanding (in shares)	19,130,643	18,824,328	18,738,905
Dilutive potential common shares outstanding:
Share-based awards (in shares)	547,525	356,151	275,490
Diluted shares outstanding (in shares)	19,678,168	19,180,479	19,014,395