FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017
Assets [Abstract]
Available-for-sale securities	$ 124,455	$ 0
Asset-backed Securities [Member]
Assets [Abstract]
Available-for-sale securities	13,901
Certificates of Deposit [Member]
Assets [Abstract]
Available-for-sale securities	2,442
Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Assets [Abstract]
Available-for-sale securities	7,754
Corporate Debt Securities [Member]
Assets [Abstract]
Available-for-sale securities	75,260
US Treasury Securities [Member]
Assets [Abstract]
Available-for-sale securities	25,098
Recurring [Member]
Assets [Abstract]
Cash and cash equivalents	186,124	397,890
Other long-term investments	1,089	929
Derivative financial instruments	8	263
Total assets	311,676	399,082
Liabilities [Abstract]
Derivative financial instruments	44	1,881
Total liabilities	44	1,881
Recurring [Member] \| Level 1 [Member]
Assets [Abstract]
Cash and cash equivalents	174,089	397,890
Other long-term investments	1,089	929
Derivative financial instruments	0	0
Total assets	175,178	398,819
Liabilities [Abstract]
Derivative financial instruments	0	0
Total liabilities	0	0
Recurring [Member] \| Level 2 [Member]
Assets [Abstract]
Cash and cash equivalents	12,035	0
Other long-term investments	0	0
Derivative financial instruments	8	263
Total assets	136,498	263
Liabilities [Abstract]
Derivative financial instruments	44	1,881
Total liabilities	44	1,881
Recurring [Member] \| Level 2 [Member] \| Asset-backed Securities [Member]
Assets [Abstract]
Available-for-sale securities	13,901
Recurring [Member] \| Level 2 [Member] \| Certificates of Deposit [Member]
Assets [Abstract]
Available-for-sale securities	2,442
Recurring [Member] \| Level 2 [Member] \| Commercial Paper, Not Included with Cash and Cash Equivalents [Member]
Assets [Abstract]
Available-for-sale securities	7,754
Recurring [Member] \| Level 2 [Member] \| Corporate Debt Securities [Member]
Assets [Abstract]
Available-for-sale securities	75,260
Recurring [Member] \| Level 2 [Member] \| US Treasury Securities [Member]
Assets [Abstract]
Available-for-sale securities	25,098
Recurring [Member] \| Level 3 [Member]
Assets [Abstract]
Cash and cash equivalents	0	0
Other long-term investments	0	0
Derivative financial instruments	0	0
Total assets	0	0
Liabilities [Abstract]
Derivative financial instruments	0	0
Total liabilities	$ 0	$ 0

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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

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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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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of long-term investments classified as other.

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