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Interim Pension and Other Postretirement Benefit Plan Information
 6 Months Ended
Mar. 31, 2025
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, 2025 and 2024 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.
 Three Months Ended March 31
 Pension BenefitsOther Benefits
 2025202420252024
 (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 BenefitsOther Benefits
2025202420252024
 (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.