Segment and Geographic Information - Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales	$ 350.8	$ 350.0	$ 340.7	$ 330.8	$ 294.5	$ 290.8	$ 270.8	$ 270.8	$ 1,372.3	$ 1,126.9	$ 970.1
Long-Lived Assets	405.1				330.3				405.1	330.3	296.5
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									418.9	375.7	349.4
Long-Lived Assets	224.0				170.3				224.0	170.3	138.3
Taiwan
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									249.1	242.8	230.8
Long-Lived Assets	38.2				38.2				38.2	38.2	36.8
South Korea
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									459.5	304.1	217.2
Long-Lived Assets	130.7				112.2				130.7	112.2	112.2
China
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									94.2	64.9	53.8
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									61.7	63.1	57.8
Long-Lived Assets	1.1				0.4				1.1	0.4	0.2
Asia, excluding China, Taiwan, and South Korea
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									88.9	76.3	61.1
Long-Lived Assets	$ 11.1				$ 9.2				$ 11.1	$ 9.2	$ 9.0