SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018
Summary of Significant Accounting Policies [Line Items]
Cash	$ 426,245	$ 347,943	$ 334,775	$ 284,355
Restricted Cash	50,000	35,000	34,300	41,800
Total cash, cash equivalents, and restricted cash	476,224	382,971	369,106	326,185
Prepaid expenses and other
Summary of Significant Accounting Policies [Line Items]
Restricted Cash	46,643	31,291	30,543	39,830
Other assets
Summary of Significant Accounting Policies [Line Items]
Restricted Cash	$ 3,336	$ 3,737	$ 3,788	$ 2,000