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Retirement and Postretirement Employee Benefit Plans - Schedule of Net Benefit Costs (Details) - USD ($)
$ in Thousands
 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2023
Employee Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other non-operating income Other non-operating income Other non-operating income
Components of net periodic postretirement cost:      
Service cost $ 10,047 $ 9,546 $ 10,805
Interest cost 23,164 25,731 24,924
Expected return on assets (30,618) (28,808) (29,113)
Amortization of prior service credit 0 0 (121)
Recognized actuarial gain 0 (67) 0
Net periodic pension cost $ 2,593 $ 6,402 $ 6,495
Supplemental Executive Retirement Plans      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other non-operating income Other non-operating income Other non-operating income
Components of net periodic postretirement cost:      
Service cost $ 1,302 $ 55 $ 845
Interest cost 3,488 4,024 4,227
Recognized actuarial gain 1,022 501 691
Settlements 0 1,529 1,030
Net periodic pension cost $ 5,812 $ 6,109 $ 6,793
Postretirement Plan      
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other non-operating income Other non-operating income Other non-operating income
Components of net periodic postretirement cost:      
Service cost $ 8,132 $ 6,028 $ 6,183
Interest cost 13,463 14,034 13,911
Expected return on assets (15,325) (12,511) (11,215)
Amortization of prior service credit (13,040) (13,040) (13,142)
Recognized actuarial gain (9,716) (10,872) (7,452)
Net periodic pension cost $ (16,486) $ (16,361) $ (11,715)