Retirement and Postretirement Employee Benefit Plans - Schedule of Net Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
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	$ 9,546	$ 10,805	$ 16,165
Interest cost	25,731	24,924	17,606
Expected return on assets	(28,808)	(29,113)	(29,531)
Amortization of prior service credit	0	(121)	(231)
Recognized actuarial (gain) loss	(67)	0	4,638
Net periodic pension cost	$ 6,402	$ 6,495	$ 8,647
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	$ 55	$ 845	$ 1,129
Interest cost	4,024	4,227	2,647
Recognized actuarial (gain) loss	501	691	3,166
Settlements	1,529	1,030	0
Net periodic pension cost	$ 6,109	$ 6,793	$ 6,942
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	$ 6,028	$ 6,183	$ 10,235
Interest cost	14,034	13,911	10,734
Expected return on assets	(12,511)	(11,215)	(13,249)
Amortization of prior service credit	(13,040)	(13,142)	(13,234)
Recognized actuarial (gain) loss	(10,872)	(7,452)	0
Net periodic pension cost	$ (16,361)	$ (11,715)	$ (5,514)