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Retirement Plan and Other Post-Retirement Benefits - Schedule of Cumulative Amounts Recognized in AOCI (Loss) and Regulatory Assets and Liabilities (Details)
$ in Thousands
12 Months Ended
Sep. 30, 2023
USD ($)
[1]
Non-Qualified Benefit Plans  
Amounts Recognized in Accumulated Other Comprehensive Income (Loss), Regulatory Assets and Regulatory Liabilities  
Net Actuarial Gain (Loss) $ (17,286)
Prior Service (Cost) Credit 0
Net Amount Recognized (17,286)
Changes to Accumulated Other Comprehensive Income (Loss), Regulatory Assets and Regulatory Liabilities  
Increase in Actuarial Gain (Loss), excluding amortization (2,139) [2]
Change due to Amortization of Actuarial (Gain) Loss 3,572
Prior Service (Cost) Credit 0
Net Change 1,433
Retirement Plan | Qualified Plan  
Amounts Recognized in Accumulated Other Comprehensive Income (Loss), Regulatory Assets and Regulatory Liabilities  
Net Actuarial Gain (Loss) (128,118)
Prior Service (Cost) Credit (2,036)
Net Amount Recognized (130,154)
Changes to Accumulated Other Comprehensive Income (Loss), Regulatory Assets and Regulatory Liabilities  
Increase in Actuarial Gain (Loss), excluding amortization (34,305) [2]
Change due to Amortization of Actuarial (Gain) Loss (7,680)
Prior Service (Cost) Credit 436
Net Change (41,549)
Other Post-Retirement Benefits | Qualified Plan  
Amounts Recognized in Accumulated Other Comprehensive Income (Loss), Regulatory Assets and Regulatory Liabilities  
Net Actuarial Gain (Loss) 18,440
Prior Service (Cost) Credit 1,115
Net Amount Recognized 19,555
Changes to Accumulated Other Comprehensive Income (Loss), Regulatory Assets and Regulatory Liabilities  
Increase in Actuarial Gain (Loss), excluding amortization 12,626 [2]
Change due to Amortization of Actuarial (Gain) Loss (8,755)
Prior Service (Cost) Credit (429)
Net Change $ 3,442
[1] Amounts presented are shown before recognizing deferred taxes.
[2] Amounts presented include the impact of actuarial gains/losses related to return on assets, as well as the Actuarial Loss amounts presented in the Change in Benefit Obligation.