FINANCIAL INFORMATION BY GEOGRAPHIC AREA - Financial Information by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 274,207	$ 274,727	$ 284,193	$ 283,143	$ 278,645	$ 271,882	$ 265,391	$ 221,778	$ 1,116,270	$ 1,037,696	$ 590,123
Property, plant and equipment, net	362,067				276,818				362,067	276,818	111,403
North America
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									399,993	372,247	79,019
Property, plant and equipment, net	250,895				133,682				250,895	133,682	60,818
Asia
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									546,866	515,833	471,215
Property, plant and equipment, net	66,872				68,823				66,872	68,823	50,573
Europe, Middle East, and Africa
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									169,099	149,305	39,889
South America
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									312	311	0
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Property, plant and equipment, net	$ 44,300				$ 74,313				$ 44,300	$ 74,313	$ 12