Retirement and Post-Retirement Employee Benefit Plans - Schedule of Net Periodic Benefit Cost Not yet Recognized (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Unrecognized prior service (credit) cost
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	$ 310	$ 251
Unrecognized prior service (credit) cost | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(815)	(1,047)
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	1,125	1,298
Unrecognized actuarial (gain) loss
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	80,193	(69,364)
Unrecognized actuarial (gain) loss | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	67,191	(2,310)
Unrecognized actuarial (gain) loss | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	56,784	33,912
Unrecognized actuarial (gain) loss | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(43,782)	(100,966)
Total Unrecognized in Net Periodic Pension Cost | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	66,376	(3,357)
Total Unrecognized in Net Periodic Pension Cost | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	56,784	33,912
Total Unrecognized in Net Periodic Pension Cost | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(42,657)	(99,668)
Total Unrecognized in Net Periodic Pension Cost | Pension and postretirement benefit costs
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	$ 80,503	$ (69,113)