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Interim Pension and Other Postretirement Benefit Plan Information (Tables)
 6 Months Ended
Mar. 31, 2024
Retirement Benefits, Description [Abstract]  
Schedule of Net Benefit Costs
 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.