FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Assets measured at fair value (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Quoted Prices in Active Markets for Identical Assets (Level 1)
Non-qualified Supplemental Savings Plan
Assets held in Non-Qualified Supplement Savings Plan, at fair value	$ 14,500	$ 16,200
Short-term investments:
Investments	45,521	$ 82,496
Recurring basis
Short-term investments:
Short-term investments	26,118
Cash and cash equivalents	243,912
Investments	45,787
Other current assets	30,363
Other assets	3,832
Total assets measured at fair value	350,012
Liabilities:
Contingent earnout liability	9,015
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Short-term investments:
Short-term investments	11,441
Cash and cash equivalents	243,912
Investments	45,521
Other current assets	30,363
Other assets	3,832
Total assets measured at fair value	335,069
Recurring basis \| Significant Other Observable Inputs (Level 2)
Short-term investments:
Short-term investments	14,677
Investments	266
Total assets measured at fair value	14,943
Recurring basis \| Significant Unobservable Inputs (Level 3)
Liabilities:
Contingent earnout liability	9,015
Certificates of deposit \| Recurring basis
Short-term investments:
Short-term investments	6,696
Certificates of deposit \| Recurring basis \| Significant Other Observable Inputs (Level 2)
Short-term investments:
Short-term investments	6,696
Corporate and municipal debt securities \| Recurring basis
Short-term investments:
Short-term investments	3,981
Corporate and municipal debt securities \| Recurring basis \| Significant Other Observable Inputs (Level 2)
Short-term investments:
Short-term investments	3,981
U.S. government and federal agency securities \| Recurring basis
Short-term investments:
Short-term investments	15,441
U.S. government and federal agency securities \| Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Short-term investments:
Short-term investments	11,441
U.S. government and federal agency securities \| Recurring basis \| Significant Other Observable Inputs (Level 2)
Short-term investments:
Short-term investments	$ 4,000

Fair value portion of other noncurrent assets.

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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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- Definition

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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-Publisher FASB
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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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- Definition

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

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

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