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Interim Pension and Other Postretirement Benefit Plan Information
 6 Months Ended
Mar. 31, 2024
Retirement Benefits, Description [Abstract]  
Interim Pension and Other Postretirement Benefit Plan Information Interim Pension and Other Postretirement Benefit Plan Information
The components of our net periodic pension cost for our pension and other postretirement benefit plans for the three and six months ended March 31, 2024 and 2023 are presented in the following tables. Most of these costs are recoverable through our tariff rates. A portion of these costs is capitalized into our rate base or deferred as a regulatory asset or liability. The remaining costs are recorded as a component of operation and maintenance expense or other non-operating expense.
In the first quarter of fiscal 2024, due to the retirement of an executive, we recognized a settlement charge of $0.8 million associated with our Supplemental Executive Retirement Plan and revalued the net periodic pension cost for the remainder of fiscal 2024. The revaluation of the net periodic pension cost for our Supplemental Executive Retirement Plan resulted in a decrease in the discount rate, effective November 30, 2023, to 5.82% from 6.17%, which will decrease our net periodic pension cost by approximately $0.4 million for the remainder of the fiscal year.
 Three Months Ended March 31
 Pension BenefitsOther Benefits
 2024202320242023
 (In thousands)
Components of net periodic pension cost:
Service cost$2,405 $2,908 $1,507 $1,546 
Interest cost (1)
7,430 7,325 3,509 3,478 
Expected return on assets (1)
(7,202)(7,278)(3,128)(2,804)
Amortization of prior service cost (credit) (1)
— (30)(3,260)(3,285)
Amortization of actuarial (gain) loss (1)
97 164 (2,718)(1,863)
Net periodic pension cost$2,730 $3,089 $(4,090)$(2,928)
 Six Months Ended March 31
 Pension BenefitsOther Benefits
2024202320242023
 (In thousands)
Components of net periodic pension cost:
Service cost$4,794 $5,816 $3,014 $3,091 
Interest cost (1)
14,926 14,650 7,017 6,955 
Expected return on assets (1)
(14,404)(14,556)(6,256)(5,608)
Amortization of prior service cost (credit) (1)
— (61)(6,520)(6,571)
Amortization of actuarial (gain) loss (1)
215 329 (5,436)(3,726)
Settlements (1)
776 — — — 
Net periodic pension cost$6,307 $6,178 $(8,181)$(5,859)
(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, 2023.