SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES, RISKS AND UNCERTAINTIES - Summary of the Retrospective Impact of the Adoption of ASU 2016-15 and ASU 2016-18 (Details) - USD ($)	12 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Consolidated Statements of Cash Flows
Change in prepaid expenses and other	$ (176,000)	$ (3,827,000)	$ 29,452,000
Change in noncurrent assets	(10,430,000)	5,568,000	11,550,000
Change in accrued liabilities	40,887,000	43,798,000	(36,120,000)
Net cash provided by operating activities	855,751,000	557,852,000	371,199,000
Payment of contingent consideration from acquisition of business	0	(10,625,000)	0
Net cash used in financing activities	$ (376,329,000)	(319,814,000)	(300,829,000)
Historical Accounting Method
Consolidated Statements of Cash Flows
Change in prepaid expenses and other		(11,218,000)	24,579,000
Change in noncurrent assets		10,263,000	6,855,000
Change in accrued liabilities		33,173,000
Net cash provided by operating activities		544,531,000	361,631,000
Payment of contingent consideration from acquisition of business		0
Net cash used in financing activities		(309,189,000)
Effect of Adoption of ASU No. 2016-15 \| Restatement Adjustment
Consolidated Statements of Cash Flows
Change in prepaid expenses and other		0	0
Change in noncurrent assets		0	0
Change in accrued liabilities		10,625,000
Net cash provided by operating activities		10,625,000	0
Payment of contingent consideration from acquisition of business		(10,625,000)
Net cash used in financing activities		(10,625,000)	0
Effect of Adoption of ASU No. 2016-18 \| Restatement Adjustment
Consolidated Statements of Cash Flows
Change in prepaid expenses and other		7,391,000	4,873,000
Change in noncurrent assets		(4,695,000)	4,695,000
Change in accrued liabilities		0
Net cash provided by operating activities		2,696,000	$ 9,568,000
Payment of contingent consideration from acquisition of business		0
Net cash used in financing activities		$ 0

The increase (decrease) during the reporting period in the aggregate amount of expenses incurred but not yet paid.

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