Earnings per Share - EPS Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net income	$ 55.3	$ 39.3	$ 74.8	$ 55.6
Less: Allocation to participating securities	(1.7)	(0.9)	(2.2)	(1.2)
Net income allocated to common stockholders	$ 53.6	$ 38.4	$ 72.6	$ 54.4
Denominator:
Common shares outstanding (in shares)	19,112,216	18,872,937	19,025,604	18,811,268
Dilutive potential common shares outstanding:
Share-based awards (in shares)	558,323	317,016	544,424	321,229
Diluted weighted - average common shares (in shares)	19,670,539	19,189,953	19,570,028	19,132,497