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	41.6				41.6
Other comprehensive income	0.3					0.3
Exercise of stock options	0.8			0.8
Share-based compensation	3.8			3.8
Repurchase of stock	0.0
Balance at end of period at Mar. 31, 2019	551.8	0.2	(46.3)	272.1	358.6	(32.8)
Balance at beginning of period at Dec. 31, 2018	526.0	0.2	(46.3)	269.7	335.2	(32.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	23.4				23.4
Other comprehensive income	0.0
Exercise of stock options	0.5			0.5
Share-based compensation	1.9			1.9
Balance at end of period at Mar. 31, 2019	551.8	0.2	(46.3)	272.1	358.6	(32.8)
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	55.6				55.6
Other comprehensive income	(2.7)					(2.7)
Exercise of stock options	4.0			4.0
Share-based compensation	5.0			5.0
Repurchase of stock	(7.5)		7.5
Balance at end of period at Mar. 31, 2020	648.6	0.2	(57.6)	285.8	458.4	(38.2)
Balance at beginning of period at Dec. 31, 2019	614.9	0.2	(50.5)	280.9	419.1	(34.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	39.3				39.3
Other comprehensive income	(3.4)					(3.4)
Exercise of stock options	2.5			2.5
Share-based compensation	2.4			2.4
Repurchase of stock	(7.1)		7.1
Balance at end of period at Mar. 31, 2020	$ 648.6	$ 0.2	$ (57.6)	$ 285.8	$ 458.4	$ (38.2)