Interim Pension and Other Postretirement Benefit Plan Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Contribution to postretirement medical plans			$ 7,400
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,652	$ 4,045	9,305	$ 8,090
Interest cost	5,843	6,801	11,686	13,600
Expected return on assets	(7,079)	(7,113)	(14,158)	(14,226)
Amortization of prior service cost (credit)	(58)	(58)	(116)	(116)
Amortization of actuarial (gain) loss	3,242	1,607	6,483	3,215
Net periodic pension cost	6,600	5,282	13,200	10,563
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3,366	2,703	6,733	5,405
Interest cost	2,653	2,958	5,306	5,919
Expected return on assets	(2,625)	(2,665)	(5,249)	(5,330)
Amortization of prior service cost (credit)	43	44	87	87
Amortization of actuarial (gain) loss	(334)	(2,044)	(669)	(4,089)
Net periodic pension cost	3,103	$ 996	6,208	$ 1,992
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