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FAIR VALUE MEASUREMENT OF FINANCIAL INSTRUMENTS (Tables) | 9 Months Ended
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Jun. 30, 2026
Fair Value Disclosures [Abstract] |
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis | 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:
| June 30, 2026
(in thousands) | Fair Value |  | Level 1 |  | Level 2 |  | Level 3
Assets |  |  |  |  |  |  |
Short-term investments: |  |  |  |  |  |  |
Money market mutual funds | $ | 5,040 |  |  | $ | 5,040 |  |  | $ | — |  |  | $ | — |
Corporate debt securities | 21,920 |  |  | — |  |  | 21,920 |  |  | — |
Total | 26,960 |  |  | 5,040 |  |  | 21,920 |  |  | — |
Long-term Investments: |  |  |  |  |  |  |
Recurring fair value measurements: |  |  |  |  |  |  |
Equity securities: |  |  |  |  |  |  |
Non-qualified supplemental savings plan | 19,196 |  |  | 19,196 |  |  | — |  |  | — |
Investment in Tamboran | 32,663 |  |  | 32,663 |  |  | — |  |  | — |
Debt securities: |  |  |  |  |  |  |
Geothermal debt securities, net | 2,000 |  |  | — |  |  | — |  |  | 2,000 |
Other debt securities | 250 |  |  | — |  |  | — |  |  | 250 |
Total | $ | 54,109 |  |  | $ | 51,859 |  |  | $ | — |  |  | $ | 2,250 |
| 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: |  |  |  |  |  |  |
Geothermal debt securities, net | 2,000 |  |  | — |  |  | — |  |  | 2,000 |
Other debt securities | 250 |  |  | — |  |  | — |  |  | 250 |
Total | $ | 47,337 |  |  | $ | 45,087 |  |  | $ | — |  |  | $ | 2,250 |
Schedule of Reconciliation of Long Term Debt Securities Available For Sale, Classified as Level 3 | The following table reconciles changes in the balance of our equity securities, without readily determinable fair values, including investments that have been marked to fair value on a nonrecurring basis, for the periods presented below:
| Three Months Ended June 30, |  | Nine Months Ended June 30,
(in thousands) | 2026 |  | 2025 |  | 2026 |  | 2025
Assets at beginning of period | $ | 17,547 |  |  | $ | 45,519 |  |  | $ | 20,861 |  |  | $ | 30,090 |
Purchases | 1,201 |  |  | 594 |  |  | 2,239 |  |  | 1,528 |
Disposals | — |  |  | (27,117) |  |  | — |  |  | (27,117) |
Sales | — |  |  | — |  |  | (124) |  |  | — |
Transfer in | — |  |  | 320 |  |  | — |  |  | 320 |
Total gain (loss): |  |  |  |  |  |  |
Included in earnings | — |  |  | 624 |  |  | (4,228) |  |  | 15,119 |
Assets at end of period | $ | 18,748 |  |  | $ | 19,940 |  |  | $ | 18,748 |  |  | $ | 19,940 |
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, net at June 30, 2026 and September 30, 2025:
| Carrying Value at June 30, 2026 |  | Fair Value at June 30, 2026 Using Inputs Considered as:
(in thousands) |  | Level 1 |  | Level 2 |  | Level 3
Unsecured senior notes: |  |  |  |  |  |  |
2027 Notes | $ | 348,446 |  |  | $ | — |  |  | $ | 349,793 |  |  | $ | — |
2029 Notes | 347,158 |  |  | — |  |  | 350,315 |  |  | — |
2031 Notes | 546,787 |  |  | — |  |  | 494,197 |  |  | — |
2034 Notes | 543,629 |  |  | — |  |  | 539,391 |  |  | — |
Secured term loan credit agreements: |  |  |  |  |  |  |
2023 Oman Facility | 32,970 |  |  | — |  |  | — |  |  | 32,970 |
2024 Oman Facility | 36,267 |  |  | — |  |  | — |  |  | 36,267 |
Total long-term debt, net of current portion | $ | 1,855,257 |  |  | $ | — |  |  | $ | 1,733,696 |  |  | $ | 69,237 |
| 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 | 35,465 |  |  | — |  |  | — |  |  | 35,465 |
2024 Oman Facility | 38,789 |  |  | — |  |  | — |  |  | 38,789 |
Total long-term debt, net of current portion | $ | 2,057,084 |  |  | $ | — |  |  | $ | 1,927,001 |  |  | $ | 74,254 |

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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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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, 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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