Earnings per Share - EPS Reconciliation (Details) - USD ($)	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net income	$ 39,300,000.0	$ 23,400,000.0	$ 55,600,000	$ 41,600,000
Less: Net income allocated to participating securities	(900,000)	(400,000)	(1,200,000)	(700,000)
Net income allocated to common shareholders	$ 38,400,000	$ 23,000,000	$ 54,400,000	$ 40,900,000
Weighted average number of common shares outstanding	18,872,937	18,753,490	18,811,268	18,706,104
Dilutive potential common shares outstanding:
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	317,016	251,303	321,229	293,785
Diluted weighted-average shares	19,189,953	19,004,793	19,132,497	18,999,889