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FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS (Tables) | 12 Months Ended
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Sep. 30, 2025
Fair Value Disclosures [Abstract] |
Schedule of Fair Value Measurements, Recurring and Nonrecurring | The following tables summarize our financial assets and liabilities measured at fair value on a recurring basis and indicate the level in the fair value hierarchy in which we classify the fair value measurement as of the dates indicated below.
| September 30, 2025
(in thousands) | Fair Value |  | Level 1 |  | Level 2 |  | Level 3
Assets |  |  |  |  |  |  |
Short-term investments: |  |  |  |  |  |  |
Corporate debt securities | $ | 21,302 |  |  | $ | — |  |  | $ | 21,302 |  |  | $ | — |
Total | 21,302 |  |  | — |  |  | 21,302 |  |  | — |
Long-term investments: |  |  |  |  |  |  |
Recurring fair value measurements: |  |  |  |  |  |  |
Equity securities: |  |  |  |  |  |  |
Non-qualified supplemental savings plan | 17,662 |  |  | 17,662 |  |  | — |  |  | — |
Investment in Tamboran | 25,976 |  |  | 25,976 |  |  | — |  |  | — |
Other equity securities | 1,449 |  |  | 1,449 |  |  | — |  |  | — |
Debt securities: |  |  |  |  |  |  |
Investment in Galileo, net | — |  |  | — |  |  | — |  |  | — |
Geothermal debt securities, net | 2,000 |  |  | — |  |  | — |  |  | 2,000 |
Other debt securities | 250 |  |  | — |  |  | — |  |  | 250 |
Total | $ | 47,337 |  |  | $ | 45,087 |  |  | $ | — |  |  | $ | 2,250 |
These investments are measured at cost, less any impairments.
| September 30, 2024
(in thousands) | Fair Value |  | Level 1 |  | Level 2 |  | Level 3
Assets |  |  |  |  |  |  |
Short-term investments: |  |  |  |  |  |  |
Corporate debt securities | $ | 33,813 |  |  | $ | — |  |  | $ | 33,813 |  |  | $ | — |
U.S. government and federal agency securities | 53,490 |  |  | 53,490 |  |  | — |  |  | — |
Investment in ADNOC Drilling | 205,616 |  |  | 205,616 |  |  | — |  |  | — |
Total | 292,919 |  |  | 259,106 |  |  | 33,813 |  |  | — |
Long-term investments: |  |  |  |  |  |  |
Recurring fair value measurements: |  |  |  |  |  |  |
Equity securities: |  |  |  |  |  |  |
Non-qualified supplemental savings plan | 15,633 |  |  | 15,633 |  |  | — |  |  | — |
Investment in Tamboran | 20,958 |  |  | 20,958 |  |  | — |  |  | — |
Debt securities: |  |  |  |  |  |  |
Investment in Galileo | 27,044 |  |  | — |  |  | — |  |  | 27,044 |
Geothermal debt securities, net | 2,000 |  |  | — |  |  | — |  |  | 2,000 |
Other debt securities | 4,588 |  |  | 4,338 |  |  | — |  |  | 250 |
Total | $ | 70,223 |  |  | $ | 40,929 |  |  | $ | — |  |  | $ | 29,294 |
Schedule of Fair Value, Debt Security Measured on Recurring Basis, Unobservable Input Reconciliation | The following table provides quantitative information (in thousands) about our Level 3 unobservable significant inputs related to our debt security investment with Galileo at September 30, 2024:
Fair Value (in thousands) |  | Valuation Technique |  | Unobservable Inputs |  |  |  |
$ | 27,044 |  |  | Black-Scholes-Merton model |  | Discount rate | 18.7 | % |  |  |  |
|  |  |  | Risk-free rate | 3.5 | % |  |  |  |
|  |  |  | Equity volatility | 66.0 | % |  |  |  |
Schedule of Reconciliation of Long Term Debt Securities Available For Sale, Classified as Level 3 | The following table reconciles changes in the fair value of our Level 3 assets for the periods presented below:
| Year Ended September 30,
(in thousands) | 2025 |  | 2024
Assets at beginning of period | $ | 29,294 |  |  | $ | 37,440 |
Purchases | — |  |  | 250 |
Accrued interest | 1,860 |  |  | 1,771 |
Total gains or (losses): |  |  |
Included in earnings | (29,287) |  |  | (10,167) |
Included in other comprehensive income (loss) | 383 |  |  | — |
Assets at end of period | $ | 2,250 |  |  | $ | 29,294 |
| Year Ended September 30,
(in thousands) | 2025 |  | 2024
Assets at beginning of period | $ | 30,090 |  |  | $ | 28,232 |
Purchases | 2,769 |  |  | 3,870 |
Disposals 1 | (27,117) |  |  | (616) |
Transfer in | 320 |  |  | — |
Total gains or (losses): |  |  |
Included in earnings 2 | 14,799 |  |  | (1,396) |
Assets at end of period | $ | 20,861 |  |  | $ | 30,090 |
(1)During the fiscal year ended September 30, 2025, we liquidated one of our geothermal equity investments for $27.1 million. (2)The gains recorded during the fiscal year ended September 30, 2025 were attributable to the change in fair value of various geothermal equity investments as a result of disposals or observable price changes in identical or similar investments during the periods.
Schedule of Supplemental Fair Value Information about Long-Term Fixed-Rate Debt | The following information presents the supplemental fair value information for our long-term fixed-rate debt at September 30, 2025 and 2024:
| Carrying Value at September 30, 2025 |  | Fair Value at September 30, 2025 Using Inputs Considered as:
(in thousands) |  | Level 1 |  | Level 2 |  | Level 3
Unsecured senior notes: |  |  |  |  |  |  |
2027 Notes | $ | 347,675 |  |  | $ | — |  |  | $ | 352,261 |  |  | $ | — |
2029 Notes | 346,602 |  |  | — |  |  | 348,688 |  |  | — |
2031 Notes | 546,336 |  |  | — |  |  | 486,343 |  |  | — |
2034 Notes | 543,197 |  |  | — |  |  | 538,417 |  |  | — |
Unsecured term loan credit agreement: |  |  |  |  |  |  |
2027 Term Loan | 199,020 |  |  | — |  |  | 201,292 |  |  | — |
Secured term loan credit agreements: |  |  |  |  |  |  |
2023 Oman Facility 1 | 35,465 |  |  | — |  |  | — |  |  | 35,465 |
2024 Oman Facility 1 | 38,789 |  |  | — |  |  | — |  |  | 38,789 |
Total long-term debt, net of current portion | $ | 2,057,084 |  |  | $ | — |  |  | $ | 1,927,001 |  |  | $ | 74,254 |
(1)The secured term credit agreements are classified as nonpublic debt, meaning their value was directly negotiated between the involved parties and is not observable in the market. As a result, they are categorized as Level 3. Since this debt is nonpublic, the carrying value and the fair value of the loans are identical.
| Carrying Value at September 30, 2024 |  | Fair Value at September 30, 2024 Using Inputs Considered as:
(in thousands) |  | Level 1 |  | Level 2 |  | Level 3
Unsecured senior notes: |  |  |  |  |  |  |
2027 Notes | $ | 347,093 |  |  | $ | — |  |  | $ | 350,700 |  |  | $ | — |
2029 Notes | 346,297 |  |  | — |  |  | 345,100 |  |  | — |
2031 Notes | 545,738 |  |  | — |  |  | 471,350 |  |  | — |
2034 Notes | 543,054 |  |  | — |  |  | 535,700 |  |  | — |
Total long-term debt | $ | 1,782,182 |  |  | $ | — |  |  | $ | 1,702,850 |  |  | $ | — |

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- Definition Tabular disclosure of financial instrument measured at fair value on recurring or nonrecurring basis. Includes, but is not limited to, instrument classified in shareholders' equity. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 100 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-100 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-3 + Details
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- Definition Tabular disclosure of input and valuation technique used to measure fair value and change in valuation approach and technique used to measure similar asset in prior period by class of asset or liability on non-recurring basis. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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- Definition Tabular disclosure of assets, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, by class that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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