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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,794	$ 5,216	$ 13,929	$ 15,649
Interest cost	6,448	6,296	19,311	18,890
Expected return on assets	(6,917)	(6,993)	(20,750)	(20,981)
Amortization of prior service cost (credit)	(57)	(57)	(173)	(173)
Amortization of actuarial (gain) loss	3,050	4,248	9,224	12,746
Settlements loss	888	0	3,303	0
Net periodic pension cost	8,206	8,710	24,844	26,131
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	3,020	3,109	9,059	9,327
Interest cost	2,726	2,669	8,180	8,009
Expected return on assets	(2,002)	(1,796)	(6,005)	(5,389)
Amortization of prior service cost (credit)	2	(411)	8	(1,233)
Amortization of actuarial (gain) loss	(1,618)	(706)	(4,855)	(2,120)
Settlements loss	0	0	0	0
Net periodic pension cost	$ 2,128	$ 2,865	$ 6,387	$ 8,594