Interim Pension and Other Postretirement Benefit Plan Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Contribution to postretirement medical plans	$ 4,100
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	4,612	$ 4,653
Interest cost	5,028	5,843
Expected return on assets	(6,978)	(7,079)
Amortization of prior service cost (credit)	(58)	(58)
Amortization of actuarial (gain) loss	3,172	(1,271)
Net periodic pension cost	5,776	2,088
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	4,306	3,366
Interest cost	2,660	2,653
Expected return on assets	(2,614)	(2,625)
Amortization of prior service cost (credit)	43	43
Amortization of actuarial (gain) loss	0	(334)
Net periodic pension cost	4,395	$ 3,103
Minimum | Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future contributions	15,000
Maximum | Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future contributions	$ 25,000