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Interim Pension and Other Postretirement Benefit Plan Information (Tables) | 6 Months Ended
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Mar. 31, 2025
Retirement Benefits, Description [Abstract] |
Schedule of Net Benefit Costs |
| Three Months Ended March 31
| Pension Benefits |  | Other Benefits
| 2025 |  | 2024 |  | 2025 |  | 2024
| (In thousands)
Components of net periodic pension cost: |  |  |  |  |  |  |
Service cost | $ | 2,837 |  |  | $ | 2,405 |  |  | $ | 2,033 |  |  | $ | 1,507 |
Interest cost (1) | 6,663 |  |  | 7,430 |  |  | 3,365 |  |  | 3,509 |
Expected return on assets (1) | (7,655) |  |  | (7,202) |  |  | (3,831) |  |  | (3,128) |
Amortization of prior service cost (credit) (1) | — |  |  | — |  |  | (3,260) |  |  | (3,260) |
Amortization of actuarial (gain) loss (1) | 256 |  |  | 97 |  |  | (2,429) |  |  | (2,718) |
Net periodic pension cost | $ | 2,101 |  |  | $ | 2,730 |  |  | $ | (4,122) |  |  | $ | (4,090) |
| Six Months Ended March 31
| Pension Benefits |  | Other Benefits
| 2025 |  | 2024 |  | 2025 |  | 2024
| (In thousands)
Components of net periodic pension cost: |  |  |  |  |  |  |
Service cost | $ | 5,674 |  |  | $ | 4,794 |  |  | $ | 4,066 |  |  | $ | 3,014 |
Interest cost (1) | 13,326 |  |  | 14,926 |  |  | 6,731 |  |  | 7,017 |
Expected return on assets (1) | (15,309) |  |  | (14,404) |  |  | (7,663) |  |  | (6,256) |
Amortization of prior service cost (credit) (1) | — |  |  | — |  |  | (6,520) |  |  | (6,520) |
Amortization of actuarial (gain) loss (1) | 511 |  |  | 215 |  |  | (4,858) |  |  | (5,436) |
Settlements (1) | — |  |  | 776 |  |  | — |  |  | — |
Net periodic pension cost | $ | 4,202 |  |  | $ | 6,307 |  |  | $ | (8,244) |  |  | $ | (8,181) |
(1) The components of net periodic cost other than the service cost component are included in the line item other non-operating expense in the condensed consolidated statements of comprehensive income or are capitalized on the condensed consolidated balance sheets as a regulatory asset or liability, as described in Note 2 to the consolidated financial statements in our Annual Report on Form 10-K for the fiscal year ended September 30, 2024.

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- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (h) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
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