Condensed Consolidated Statement of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss, net
Balance at beginning of period at Sep. 30, 2018	$ 505.3	$ 0.2	$ 46.3	$ 267.5	$ 317.0	$ (33.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	57.9				57.9
Other comprehensive income	0.6					0.6
Exercise of stock options	0.8			0.8
Share-based compensation	5.9			5.9
Repurchase of stock	0.0
Balance at end of period at Jun. 30, 2019	570.5	0.2	(46.3)	274.2	374.9	(32.5)
Balance at beginning of period at Mar. 31, 2019	551.8	0.2	(46.3)	272.1	358.6	(32.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	16.3				16.3
Other comprehensive income	0.3
Share-based compensation	2.1			2.1
Balance at end of period at Jun. 30, 2019	570.5	0.2	(46.3)	274.2	374.9	(32.5)
ASU 2018-02 cumulative transition adjustment	0.7				0.7	(0.7)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	594.2	0.2	(50.1)	276.8	402.8	(35.5)
Balance at beginning of period at Sep. 30, 2019	594.2	0.2	(50.1)	276.8	402.1	(34.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	92.2				92.2
Other comprehensive income	(2.7)					(2.7)
Exercise of stock options	5.2			5.2
Share-based compensation	7.4			7.4
Repurchase of stock	(7.5)		7.5
Balance at end of period at Jun. 30, 2020	688.8	0.2	(57.6)	289.4	495.0	(38.2)
Balance at beginning of period at Mar. 31, 2020	648.6	0.2	(57.6)	285.8	458.4	(38.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	36.6				36.6
Other comprehensive income	0.0
Exercise of stock options	1.2			1.2
Share-based compensation	2.4			2.4
Balance at end of period at Jun. 30, 2020	$ 688.8	$ 0.2	$ (57.6)	$ 289.4	$ 495.0	$ (38.2)