Retirement and Post-Retirement Employee Benefit Plans - Schedule of Net Periodic Benefit Cost Not yet Recognized (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Unrecognized prior service (credit) cost
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	$ 367	$ 310
Unrecognized prior service (credit) cost | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	(584)	(815)
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	951	1,125
Unrecognized actuarial loss
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	138,237	80,193
Unrecognized actuarial loss | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	78,082	67,191
Unrecognized actuarial loss | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	51,045	56,784
Unrecognized actuarial loss | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	9,110	(43,782)
Total Unrecognized in Net Periodic Pension Cost | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	77,498	66,376
Total Unrecognized in Net Periodic Pension Cost | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	51,045	56,784
Total Unrecognized in Net Periodic Pension Cost | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	10,061	(42,657)
Total Unrecognized in Net Periodic Pension Cost | Pension and postretirement benefit costs
Defined Benefit Plan Disclosure [Line Items]
Net Regulatory Assets	$ 138,604	$ 80,503