Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Restricted Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 487,884	$ 347,943	$ 284,355
Restricted Cash
Restricted cash and cash equivalents	48,900	35,000
Total cash, cash equivalents, and restricted cash	536,747	382,971	326,185	$ 560,509
Prepaid expenses and other
Restricted Cash
Restricted cash and cash equivalents	45,577	31,291	39,830
Other assets
Restricted Cash
Restricted cash and cash equivalents	$ 3,286	$ 3,737	$ 2,000