FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS - Long-term debt fair value (Details) - USD ($) $ in Thousands	6 Months Ended
Mar. 31, 2019	Sep. 30, 2018	Mar. 19, 2015
Supplemental fair value information about long-term fixed-rate debt
Long-term debt	$ 491,227	$ 493,968
Debt issued	500,000	500,000
Decrease in fair value of investments	37,000
HPIDC 2025 Notes
Supplemental fair value information about long-term fixed-rate debt
Debt issued	500,000	500,000	$ 500,000
Significant Other Observable Inputs (Level 2)
Supplemental fair value information about long-term fixed-rate debt
Fair value of long-term fixed-rate debt	507,000	509,300
Carrying value
Supplemental fair value information about long-term fixed-rate debt
Long-term debt	$ 491,200	$ 494,000

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

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- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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