Interim Pension and Other Postretirement Benefit Plan Information - Schedule of Net Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,044	$ 4,794	$ 12,134	$ 13,929
Interest cost	6,799	6,448	20,399	19,311
Expected return on assets	(7,113)	(6,917)	(21,339)	(20,750)
Amortization of prior service cost (credit)	(57)	(57)	(173)	(173)
Amortization of actuarial (gain) loss	1,606	3,050	4,821	9,224
Settlements	0	(888)	0	(3,303)
Net periodic pension cost	5,279	8,206	15,842	24,844
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	2,702	3,020	8,107	9,059
Interest cost	2,960	2,726	8,879	8,180
Expected return on assets	(2,664)	(2,002)	(7,994)	(6,005)
Amortization of prior service cost (credit)	43	2	130	8
Amortization of actuarial (gain) loss	(2,045)	(1,618)	(6,134)	(4,855)
Settlements	0	0	0	0
Net periodic pension cost	$ 996	$ 2,128	$ 2,988	$ 6,387