Condensed Consolidated Statement of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Treasury Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Balances as of beginning of period at Sep. 30, 2017	$ 449.9	$ 0.2	$ 46.3	$ 259.0	$ 261.5	$ (24.5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	39.8				39.8
Other comprehensive loss	(8.7)					(8.7)
Stock Issued During Period, Value, Stock Options Exercised	1.8			1.8
Share-based Compensation	4.9			4.9
Balances as of end of period at Jun. 30, 2018	487.7	0.2	46.3	265.7	301.3	(33.2)
Balances as of beginning of period at Mar. 31, 2018	466.6	0.2	46.3	263.7	277.3	(28.3)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	24.0				24.0
Other comprehensive loss	(4.9)					(4.9)
Stock Issued During Period, Value, Stock Options Exercised	0.3			0.3
Share-based Compensation	1.7			1.7
Balances as of end of period at Jun. 30, 2018	487.7	0.2	46.3	265.7	301.3	(33.2)
Balances as of 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 loss	0.6					0.6
Stock Issued During Period, Value, Stock Options Exercised	0.8			0.8
Share-based Compensation	5.9			5.9
Balances as of end of period at Jun. 30, 2019	570.5	0.2	(46.3)	274.2	374.9	(32.5)
Balances as of 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 loss	0.3					0.3
Share-based Compensation	2.1			2.1
Balances as of end of period at Jun. 30, 2019	$ 570.5	$ 0.2	$ (46.3)	$ 274.2	$ 374.9	$ (32.5)