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Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands | Dec. 19, 2024 | Sep. 30, 2025 | Sep. 30, 2024
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Regulatory Assets: |  |  |  |
Total Long-Term Regulatory Assets |  |  | $ 230,969 | $ 193,710
Total Regulatory Assets |  |  | 272,100 | 231,974
Regulatory Liabilities: |  |  |  |
Total Long-Term Regulatory Liabilities |  |  | 735,938 | 749,574
Total Regulatory Liabilities |  |  | 757,530 | 821,646
Levelization deferral |  | $ 17,800 |  |
Uncollectible expense tracker |  | 9,800 |  |
Property tax tracker |  | $ 1,100 |  |
Taxes Refundable to Customers |  |  |  |
Regulatory Liabilities: |  |  |  |
Total Long-Term Regulatory Liabilities |  |  | 306,335 | 305,645
Cost of Removal Regulatory Liability |  |  |  |
Regulatory Liabilities: |  |  |  |
Total Long-Term Regulatory Liabilities |  |  | 307,659 | 292,477
Post-Retirement Benefit Costs |  |  |  |
Regulatory Liabilities: |  |  |  |
Total Long-Term Regulatory Liabilities |  |  | 108,714 | 135,399
Current Regulatory Liabilities |  |  | 5,800 | 5,800
Environmental Site Remediation Costs |  |  |  |
Regulatory Liabilities: |  |  |  |
Total Long-Term Regulatory Liabilities |  |  | 1,800 | 5,390
Other |  |  |  |
Regulatory Liabilities: |  |  |  |
Total Long-Term Regulatory Liabilities |  |  | 11,430 | 10,663
Amounts Payable to Customers |  |  |  |
Regulatory Liabilities: |  |  |  |
Current Regulatory Liabilities |  |  | 968 | 42,720
Other Current and Accrued Liabilities |  |  |  |
Regulatory Liabilities: |  |  |  |
Current Regulatory Liabilities |  |  | 14,824 | 23,552
Recoverable Future Taxes |  |  |  |
Regulatory Assets: |  |  |  |
Total Long-Term Regulatory Assets |  |  | 89,247 | 80,084
Unamortized Debt Expense |  |  |  |
Regulatory Assets: |  |  |  |
Total Long-Term Regulatory Assets |  |  | 6,236 | 5,604
Pension Costs |  |  |  |
Regulatory Assets: |  |  |  |
Total Long-Term Regulatory Assets |  |  | 42,602 | 30,259
NY Rate Case Levelization and Tracking Mechanisms |  |  |  |
Regulatory Assets: |  |  |  |
Total Long-Term Regulatory Assets | [1] |  | 28,688 | 0
Asset Retirement Obligations |  |  |  |
Regulatory Assets: |  |  |  |
Total Long-Term Regulatory Assets |  |  | 24,634 | 21,951
Post-Retirement Benefit Costs |  |  |  |
Regulatory Assets: |  |  |  |
Total Long-Term Regulatory Assets |  |  | 13,306 | 7,932
System Modernization/Improvement Tracker |  |  |  |
Regulatory Assets: |  |  |  |
Total Long-Term Regulatory Assets |  |  | 12,625 | 31,362
Current Regulatory Assets |  |  | 14,481 | 15,681
Other |  |  |  |
Regulatory Assets: |  |  |  |
Total Long-Term Regulatory Assets |  |  | 13,631 | 16,518
Current Regulatory Assets |  |  | 20,881 | 22,583
Unrecovered Purchased Gas Costs |  |  |  |
Regulatory Assets: |  |  |  |
Current Regulatory Assets |  |  | $ 5,769 | $ 0
[1] | New York Rate Case Levelization and Tracking Mechanisms were approved in accordance with Distribution Corporation’s New York rate settlement on December 19, 2024, as discussed below. The mechanisms include: (a) levelization deferral of $17.8 million which relates to a volumetric surcredit within the Company’s delivery adjustment charge that minimizes customer bill impacts, (b) uncollectible expense tracker of $9.8 million, which tracks and reconciles the actual uncollectible expense to the amounts recovered in base rates and, (c) property tax tracker of $1.1 million, which represents the amount deferred between actual property taxes incurred and the level included in customer rates.

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 980 -SubTopic 340 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478742/980-340-50-1 + Details
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