SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Summary of Significant Accounting Policies [Line Items]
Cash and cash equivalents	$ 373,980	$ 487,884	$ 355,010	$ 347,943
Restricted cash	48,700	48,900	39,300	35,000
Total cash, cash equivalents, and restricted cash	422,653	536,747	394,315	382,971
Prepaid expenses and other, net
Summary of Significant Accounting Policies [Line Items]
Restricted cash	45,688	45,577	35,618	31,291
Other assets, net
Summary of Significant Accounting Policies [Line Items]
Restricted cash	$ 2,985	$ 3,286	$ 3,687	$ 3,737