DEBT - Components of Unsecured Long-Term Debt Outstanding (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Sep. 30, 2020
Long-term debt, gross
Face Amount	$ 487,148	$ 487,148
Unamortized Discount and Debt Issuance Cost	(6,146)	(6,421)
Book Value	481,002	480,727
Less long-term debt due within one year
Face Amount	0	0
Unamortized Discount and Debt Issuance Cost	0	0
Book Value	0	0
Long-term debt
Face Amount	487,148	487,148
Unamortized Discount and Debt Issuance Cost	(6,146)	(6,421)
Book Value	481,002	480,727
HPIDC 2025 Notes
Long-term debt, gross
Face Amount	487,148	487,148
Unamortized Discount and Debt Issuance Cost	(6,146)	(6,421)
Book Value	481,002	480,727
Long-term debt
Face Amount	$ 487,148	$ 487,148

Debt Instrument, Unamortized Discount (Premium) And Debt Issuance Costs, Net, Current

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Debt Instrument, Unamortized Discount (Premium) And Debt Issuance Costs, Net, Noncurrent

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Long-term Debt, Current Maturities, Gross

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Amount of unamortized debt discount (premium) and debt issuance costs.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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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.

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