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	15.8				15.8
Other comprehensive loss	(3.8)					(3.8)
Stock Issued During Period, Value, Stock Options Exercised	1.5			1.5
Share-based Compensation	3.2			3.2
Balances as of end of period at Mar. 31, 2018	466.6	0.2	46.3	263.7	277.3	(28.3)
Balances as of beginning of period at Dec. 31, 2017	443.2	0.2	46.3	261.4	254.6	(26.7)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	22.7				22.7
Other comprehensive loss	(1.6)					(1.6)
Stock Issued During Period, Value, Stock Options Exercised	0.7			0.7
Share-based Compensation	1.6			1.6
Balances as of end of period at Mar. 31, 2018	466.6	0.2	46.3	263.7	277.3	(28.3)
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	41.6				41.6
Other comprehensive loss	0.3					0.3
Stock Issued During Period, Value, Stock Options Exercised	0.8			0.8
Share-based Compensation	3.8			3.8
Balances as of end of period at Mar. 31, 2019	551.8	0.2	(46.3)	272.1	358.6	(32.8)
Balances as of 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 loss	0.0
Stock Issued During Period, Value, Stock Options Exercised	0.5			0.5
Share-based Compensation	1.9			1.9
Balances as of end of period at Mar. 31, 2019	$ 551.8	$ 0.2	$ (46.3)	$ 272.1	$ 358.6	$ (32.8)