FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Summary of Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Liabilities:
Contingent earnout liability	$ 4,197	$ 12,838
Recurring Fair Value Measurements
Assets:
Short-term investments	89,335
Cash and cash equivalents	487,884
Investments	11,766
Other current assets	45,577
Other assets	3,286
Total assets measured at fair value	637,848
Liabilities:
Contingent earnout liability	9,123
Level 1 | Recurring Fair Value Measurements
Assets:
Short-term investments	9,809
Cash and cash equivalents	487,884
Investments	7,274
Other current assets	45,577
Other assets	3,286
Total assets measured at fair value	553,830
Liabilities:
Contingent earnout liability	0
Level 2 | Recurring Fair Value Measurements
Assets:
Short-term investments	79,526
Cash and cash equivalents	0
Investments	3,992
Other current assets	0
Other assets	0
Total assets measured at fair value	83,518
Liabilities:
Contingent earnout liability	0
Level 3 | Recurring Fair Value Measurements
Assets:
Short-term investments	0
Cash and cash equivalents	0
Investments	500
Other current assets	0
Other assets	0
Total assets measured at fair value	500
Liabilities:
Contingent earnout liability	9,123
Certificates of deposit | Recurring Fair Value Measurements
Assets:
Short-term investments	1,370
Certificates of deposit | Level 1 | Recurring Fair Value Measurements
Assets:
Short-term investments	0
Certificates of deposit | Level 2 | Recurring Fair Value Measurements
Assets:
Short-term investments	1,370
Certificates of deposit | Level 3 | Recurring Fair Value Measurements
Assets:
Short-term investments	0
Corporate and municipal debt securities | Recurring Fair Value Measurements
Assets:
Short-term investments	78,156
Corporate and municipal debt securities | Level 1 | Recurring Fair Value Measurements
Assets:
Short-term investments	0
Corporate and municipal debt securities | Level 2 | Recurring Fair Value Measurements
Assets:
Short-term investments	78,156
Corporate and municipal debt securities | Level 3 | Recurring Fair Value Measurements
Assets:
Short-term investments	0
U.S. government and federal agency securities | Recurring Fair Value Measurements
Assets:
Short-term investments	7,817
U.S. government and federal agency securities | Level 1 | Recurring Fair Value Measurements
Assets:
Short-term investments	7,817
U.S. government and federal agency securities | Level 2 | Recurring Fair Value Measurements
Assets:
Short-term investments	0
U.S. government and federal agency securities | Level 3 | Recurring Fair Value Measurements
Assets:
Short-term investments	0
Other | Recurring Fair Value Measurements
Assets:
Short-term investments	1,992
Other | Level 1 | Recurring Fair Value Measurements
Assets:
Short-term investments	1,992
Other | Level 2 | Recurring Fair Value Measurements
Assets:
Short-term investments	0
Other | Level 3 | Recurring Fair Value Measurements
Assets:
Short-term investments	$ 0