Segment and Geographic Information - Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales	$ 294.5	$ 290.8	$ 270.8	$ 270.8	$ 248.4	$ 242.7	$ 233.5	$ 245.5	$ 1,126.9	$ 970.1	$ 1,009.3
Long-Lived Assets	330.3				296.5				330.3	296.5	301.1
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									375.7	349.4	361.3
Long-Lived Assets	170.3				138.3				170.3	138.3	139.9
Taiwan
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									242.8	230.8	236.3
Long-Lived Assets	38.2				36.8				38.2	36.8	39.7
South Korea
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									304.1	217.2	220.3
Long-Lived Assets	112.2				112.2				112.2	112.2	105.0
China
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									64.9	53.8	70.9
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									63.1	57.8	69.2
Long-Lived Assets	0.4				0.2				0.4	0.2	0.1
Asia, excluding China, Taiwan, and South Korea
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									76.3	61.1	51.3
Long-Lived Assets	$ 9.2				$ 9.0				$ 9.2	$ 9.0	$ 16.4