Retirement and Post-Retirement Employee Benefit Plans - Schedule of Net Periodic Benefit Cost Not yet Recognized (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Unrecognized prior service (credit) cost
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	$ 251	$ 31
Unrecognized prior service (credit) cost | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(1,047)	(1,278)
Unrecognized prior service (credit) cost | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	0	0
Unrecognized prior service (credit) cost | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	1,298	1,309
Unrecognized actuarial (gain) loss
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(69,364)	17,362
Unrecognized actuarial (gain) loss | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(2,310)	62,388
Unrecognized actuarial (gain) loss | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	33,912	42,170
Unrecognized actuarial (gain) loss | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(100,966)	(87,196)
Total Unrecognized in Net Periodic Pension Cost | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(3,357)	61,110
Total Unrecognized in Net Periodic Pension Cost | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	33,912	42,170
Total Unrecognized in Net Periodic Pension Cost | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(99,668)	(85,887)
Total Unrecognized in Net Periodic Pension Cost | Pension and postretirement benefit costs
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	$ (69,113)	$ 17,393