SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 427,243	$ 487,884	$ 336,089	$ 347,943
Restricted cash	51,400	48,900	44,100	35,000
Total cash, cash equivalents, and restricted cash	478,635	536,747	380,219	382,971
Prepaid expenses and other, net
Summary of Significant Accounting Policies [Line Items]
Restricted cash	48,457	45,577	40,744	31,291
Other assets, net
Summary of Significant Accounting Policies [Line Items]
Restricted cash	$ 2,935	$ 3,286	$ 3,386	$ 3,737