FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Assets Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Sep. 30, 2018
(Level 1)
Short-term investments:
Investments	$ 32,226	$ 82,496
Recurring basis
Short-term investments:
Short-term investments	45,748
Cash and cash equivalents	334,775
Investments	32,517
Other current assets	30,543
Other assets	3,788
Total assets measured at fair value	447,371
Liabilities:
Contingent earnout liability	9,555
Recurring basis \| (Level 1)
Short-term investments:
Short-term investments	26,840
Cash and cash equivalents	334,775
Investments	32,226
Other current assets	30,543
Other assets	3,788
Total assets measured at fair value	428,172
Liabilities:
Contingent earnout liability	0
Recurring basis \| (Level 2)
Short-term investments:
Short-term investments	18,908
Cash and cash equivalents	0
Investments	291
Other current assets	0
Other assets	0
Total assets measured at fair value	19,199
Liabilities:
Contingent earnout liability	0
Recurring basis \| (Level 3)
Short-term investments:
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	9,555
Certificates of deposit \| Recurring basis
Short-term investments:
Short-term investments	6,728
Certificates of deposit \| Recurring basis \| (Level 1)
Short-term investments:
Short-term investments	0
Certificates of deposit \| Recurring basis \| (Level 2)
Short-term investments:
Short-term investments	6,728
Certificates of deposit \| Recurring basis \| (Level 3)
Short-term investments:
Short-term investments	0
Corporate and municipal debt securities \| Recurring basis
Short-term investments:
Short-term investments	12,180
Corporate and municipal debt securities \| Recurring basis \| (Level 1)
Short-term investments:
Short-term investments	0
Corporate and municipal debt securities \| Recurring basis \| (Level 2)
Short-term investments:
Short-term investments	12,180
Corporate and municipal debt securities \| Recurring basis \| (Level 3)
Short-term investments:
Short-term investments	0
U.S. government and federal agency securities \| Recurring basis
Short-term investments:
Short-term investments	26,840
U.S. government and federal agency securities \| Recurring basis \| (Level 1)
Short-term investments:
Short-term investments	26,840
U.S. government and federal agency securities \| Recurring basis \| (Level 2)
Short-term investments:
Short-term investments	0
U.S. government and federal agency securities \| Recurring basis \| (Level 3)
Short-term investments:
Short-term investments	$ 0

Fair value portion of other noncurrent assets.

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- Definition

Represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with fair value disclosures required in footnote disclosures to financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of assets categorized as other current assets which are not in and of themselves material enough to require separate disclosure.

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Fair value portion of short-term investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of liability recognized arising from contingent consideration in a business combination, expected to be settled beyond one year or the normal operating cycle, if longer.

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-Publisher FASB
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes 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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- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
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