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Regulatory assets and liabilities - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands | Sep. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024
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Regulatory Asset [Abstract] |  |  |
Current regulatory assets | $ 179,149 | $ 215,436 | $ 241,008
Regulatory assets | 305,890 | 322,350 | 343,578
Total regulatory assets | 485,039 | 537,786 | 584,586
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 143,822 | 137,167 | 157,068
Regulatory liabilities | 473,812 | 459,170 | 463,790
Total regulatory liabilities | 617,634 | 596,337 | 620,858
Net regulatory position | $ (132,595) | (58,551) | (36,272)
Environmental compliance programs |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 84,884 | 72,387 | 77,392
Natural gas costs refundable through rate adjustments |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 43,373 | 45,427 | 54,197
Provision for rate refund |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 4,358 | 3,677 | 8,868
Margin sharing |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 3,831 | 4,156 | 3,945
Taxes recoverable from customers |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 2,325 | 2,163 | 2,195
Regulatory liabilities | $ 177,469 | 185,402 | 184,715
Conservation programs |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 1,108 | 2,082 | 2,662
Cost recovery mechanisms |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 154 | 1,720 | 2,634
Regulatory liabilities | 38,426 | 30,354 | 28,397
Other |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 3,789 | 5,555 | 5,175
Regulatory liabilities | 2,458 | 2,161 | 2,117
Plant removal and decommissioning costs |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | 228,982 | 217,603 | 224,839
Accumulated deferred investment tax credit |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | 21,615 | 18,788 | 17,679
Pension and postretirement benefits |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | $ 4,862 | 4,862 | 6,043
Minimum | Cost recovery mechanisms |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Minimum | Other |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Maximum | Cost recovery mechanisms |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 17 years |  |
Maximum | Other |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 13 years |  |
Environmental compliance programs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 69,646 | 76,964 | 80,198
Regulatory assets | $ 0 | 0 | 2,858
Natural gas costs recoverable through rate adjustments |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 34,325 | 91,091 | 111,946
Regulatory assets | $ 0 | 0 | 11,707
Conservation programs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 27,405 | 19,123 | 19,059
Demand cost deferral |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 15,287 | 5,237 | 13,898
Decoupling mechanisms |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 9,716 | 6,767 | 4,795
Electric fuel and purchased power deferral |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 9,644 | 9,662 | 4,497
Regulatory assets | 0 | 4,349 | 8,335
Cost recovery mechanisms |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | 8,606 | 5,114 | 4,332
Regulatory assets | $ 68,314 | 76,542 | 82,907
Cost recovery mechanisms | Minimum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Cost recovery mechanisms | Maximum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 24 years |  |
Other |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | $ 4,520 | 1,478 | 2,283
Regulatory assets | $ 4,340 | 5,990 | 5,890
Other | Minimum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Other | Maximum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 14 years |  |
Pension and postretirement benefits |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | $ 142,064 | 142,064 | 142,511
Plant costs/asset retirement obligations |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | 46,334 | 47,042 | 45,249
Manufactured gas plant site remediation |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | 27,147 | 27,964 | 25,789
Taxes recoverable from customers |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | $ 12,257 | 12,221 | 12,316
Covid-19 deferred costs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 3 years |  |
Regulatory assets | $ 3,863 | 4,167 | 3,857
Long-term debt refinancing costs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 36 years |  |
Regulatory assets | $ 1,571 | $ 2,011 | $ 2,159

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- Definition The net amount of all regulatory assets less all regulatory liabilities as of the end of the period. + 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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- Definition Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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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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