Interim Pension and Other Postretirement Benefit Plan Information - Schedule of Components of Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,405		$ 2,908	$ 4,794	$ 5,816
Interest cost	7,430		7,325	14,926	14,650
Expected return on assets	(7,202)		(7,278)	(14,404)	(14,556)
Amortization of prior service cost (credit)	0		(30)	0	(61)
Amortization of actuarial (gain) loss	97		164	215	329
Settlements		$ (800)		(776)	0
Net periodic pension cost	2,730		3,089	6,307	6,178
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,507		1,546	3,014	3,091
Interest cost	3,509		3,478	7,017	6,955
Expected return on assets	(3,128)		(2,804)	(6,256)	(5,608)
Amortization of prior service cost (credit)	(3,260)		(3,285)	(6,520)	(6,571)
Amortization of actuarial (gain) loss	(2,718)		(1,863)	(5,436)	(3,726)
Settlements				0	0
Net periodic pension cost	$ (4,090)		$ (2,928)	$ (8,181)	$ (5,859)