Interim Pension and Other Postretirement Benefit Plan Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Contribution to postretirement medical plans			$ 12,200
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,652	$ 4,044	13,957	$ 12,134
Interest cost	5,843	6,799	17,529	20,399
Expected return on assets	(7,079)	(7,113)	(21,237)	(21,339)
Amortization of prior service cost (credit)	(58)	(57)	(174)	(173)
Amortization of actuarial (gain) loss	3,242	1,606	9,725	4,821
Net periodic pension cost	6,600	5,279	19,800	15,842
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3,366	2,702	10,099	8,107
Interest cost	2,653	2,960	7,959	8,879
Expected return on assets	(2,625)	(2,664)	(7,874)	(7,994)
Amortization of prior service cost (credit)	43	43	130	130
Amortization of actuarial (gain) loss	(334)	(2,045)	(1,003)	(6,134)
Net periodic pension cost	3,103	$ 996	9,311	$ 2,988
Minimum | Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future contributions	10,000		10,000
Maximum | Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Expected future contributions	$ 20,000		$ 20,000