Earnings per Share - EPS Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income	$ 169.6	$ 85.1	$ 55.5
Less: Allocation to participating securities	(4.0)	(1.5)	(0.9)
Net income allocated to common stockholders	$ 165.6	$ 83.6	$ 54.6
Denominator:
Common shares outstanding (in shares)	18,824,328	18,738,905	18,549,011
Dilutive potential common shares outstanding:
Share-based awards (in shares)	356,151	275,490	385,819
Diluted shares outstanding (in shares)	19,180,479	19,014,395	18,934,830