FINANCIAL INFORMATION BY GEOGRAPHIC AREA - Revenue by Major Foreign Customers (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
South Korea | Customer concentration risk | Revenue benchmark
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									127,972	135,844	136,403
Taiwan | Customer concentration risk | Revenue benchmark
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									133,059	$ 125,895	130,500
China | Customer concentration risk | Revenue benchmark
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue									$ 113,570		$ 97,254