Interim Pension and Other Postretirement Benefit Plan Information - Schedule of Net Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,045	$ 4,575	$ 8,090	$ 9,135
Interest cost	6,801	6,433	13,600	12,863
Expected return on assets	(7,113)	(6,916)	(14,226)	(13,833)
Amortization of prior service cost (credit)	(58)	(58)	(116)	(116)
Amortization of actuarial (gain) loss	1,607	3,085	3,215	6,174
Settlements	0	(2,415)	0	(2,415)
Net periodic pension cost	5,282	9,534	10,563	16,638
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,703	3,019	5,405	6,039
Interest cost	2,958	2,727	5,919	5,454
Expected return on assets	(2,665)	(2,001)	(5,330)	(4,003)
Amortization of prior service cost (credit)	44	3	87	6
Amortization of actuarial (gain) loss	(2,044)	(1,619)	(4,089)	(3,237)
Settlements	0	0	0	0
Net periodic pension cost	$ 996	$ 2,129	$ 1,992	$ 4,259