Condensed Consolidated Statement of Stockholders' Equity - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Treasury Stock	Treasury Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Additional Paid-in Capital	Additional Paid-in Capital Cumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated Other Comprehensive Loss, net	Accumulated Other Comprehensive Loss, net Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Loss, net Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning balance at Sep. 30, 2019	$ 594.2	$ 0.0	$ 594.2	$ 0.2	$ 0.2	$ (50.1)	$ (50.1)	$ 276.8	$ 276.8	$ 402.1	$ 0.7	$ 402.8	$ (34.8)	$ (0.7)	$ (35.5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	55.6									55.6
Other comprehensive income (loss)	(2.7)												(2.7)
Exercise of stock options	4.0							4.0
Share-based compensation	5.0							5.0
Repurchase of common stock	(7.5)					(7.5)
Ending balance at Mar. 31, 2020	648.6			0.2		(57.6)		285.8		458.4			(38.2)
Beginning balance at Sep. 30, 2019	594.2	0.0	594.2	0.2	0.2	(50.1)	(50.1)	276.8	276.8	402.1	0.7	402.8	(34.8)	$ (0.7)	(35.5)
Ending balance at Sep. 30, 2020	$ 767.5	(6.2)	761.3	0.2	0.2	(57.6)	(57.6)	292.6	292.6	572.4	(6.2)	566.2	(40.1)		(40.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Beginning balance 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 (loss)	(3.4)												(3.4)
Exercise of stock options	2.5							2.5
Share-based compensation	2.4							2.4
Repurchase of common stock	(7.1)					(7.1)
Ending balance at Mar. 31, 2020	648.6			0.2		(57.6)		285.8		458.4			(38.2)
Beginning balance at Sep. 30, 2020	767.5	$ (6.2)	$ 761.3	0.2	$ 0.2	(57.6)	$ (57.6)	292.6	$ 292.6	572.4	$ (6.2)	$ 566.2	(40.1)		$ (40.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	74.8									74.8
Other comprehensive income (loss)	12.3												12.3
Exercise of stock options	5.3							5.3
Share-based compensation	6.5							6.5
Ending balance at Mar. 31, 2021	860.2			0.2		(57.6)		304.4		641.0			(27.8)
Beginning balance at Dec. 31, 2020	799.5			0.2		(57.6)		297.4		585.7			(26.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	55.3									55.3
Other comprehensive income (loss)	(1.6)												(1.6)
Exercise of stock options	3.6							3.6
Share-based compensation	3.4							3.4
Ending balance at Mar. 31, 2021	$ 860.2			$ 0.2		$ (57.6)		$ 304.4		$ 641.0			$ (27.8)