FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Summary of Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Liabilities:
Contingent earnout liability	$ 12,838	$ 11,160
Recurring Fair Value Measurements
Assets:
Short-term investments	52,960
Cash and cash equivalents	347,943
Investments	16,267
Other current assets	31,291
Other assets	3,737
Total assets measured at fair value	452,198
Liabilities:
Contingent earnout liability	18,373
Level 1 | Recurring Fair Value Measurements
Assets:
Short-term investments	28,386
Cash and cash equivalents	347,943
Investments	15,974
Other current assets	31,291
Other assets	3,737
Total assets measured at fair value	427,331
Liabilities:
Contingent earnout liability	0
Level 2 | Recurring Fair Value Measurements
Assets:
Short-term investments	24,574
Cash and cash equivalents	0
Investments	293
Other current assets	0
Other assets	0
Total assets measured at fair value	24,867
Liabilities:
Contingent earnout liability	0
Level 3 | Recurring Fair Value Measurements
Assets:
Short-term investments	0
Cash and cash equivalents	0
Investments	0
Other current assets	0
Other assets	0
Total assets measured at fair value	0
Liabilities:
Contingent earnout liability	18,373
Certificates of deposit | Recurring Fair Value Measurements
Assets:
Short-term investments	6,590
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	6,590
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	17,984
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	17,984
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	28,386
U.S. government and federal agency securities | Level 1 | Recurring Fair Value Measurements
Assets:
Short-term investments	28,386
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