Retirement and Post-Retirement Employee Benefit Plans - Schedule of Net Periodic Benefit Cost Not yet Recognized (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Sep. 30, 2020
Defined Benefit Plan | Domestic equities
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	44.50%	45.30%
Defined Benefit Plan | Domestic equities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	35.00%
Defined Benefit Plan | Domestic equities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	55.00%
Defined Benefit Plan | International equities
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	16.90%	15.60%
Defined Benefit Plan | International equities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	10.00%
Defined Benefit Plan | International equities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	20.00%
Defined Benefit Plan | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	16.00%	17.00%
Defined Benefit Plan | Fixed income | Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	5.00%
Defined Benefit Plan | Fixed income | Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	30.00%
Defined Benefit Plan | Company stock
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	10.60%	13.00%
Defined Benefit Plan | Company stock | Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	0.00%
Defined Benefit Plan | Company stock | Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	15.00%
Defined Benefit Plan | Other assets
Defined Benefit Plan Disclosure [Line Items]
Actual allocation	12.00%	9.10%
Defined Benefit Plan | Other assets | Minimum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	0.00%
Defined Benefit Plan | Other assets | Maximum
Defined Benefit Plan Disclosure [Line Items]
Targeted Allocation  Range	20.00%
Unrecognized prior service (credit) cost
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	$ (64,666)	$ 367
Unrecognized prior service (credit) cost | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(353)	(584)
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	(64,313)	951
Unrecognized actuarial (gain) loss
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	8,465	138,237
Unrecognized actuarial (gain) loss | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(3,060)	78,082
Unrecognized actuarial (gain) loss | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	39,666	51,045
Unrecognized actuarial (gain) loss | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(28,141)	9,110
Total Unrecognized in Net Periodic Pension Cost | Defined Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(3,413)	77,498
Total Unrecognized in Net Periodic Pension Cost | Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	39,666	51,045
Total Unrecognized in Net Periodic Pension Cost | Postretirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	(92,454)	10,061
Total Unrecognized in Net Periodic Pension Cost | Pension and postretirement benefit costs
Defined Benefit Plan Disclosure [Line Items]
Net regulatory assets	$ (56,201)	$ 138,604