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Regulatory assets and liabilities - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands | Jun. 30, 2025 | Dec. 31, 2024 | Jun. 30, 2024
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Regulatory Asset [Abstract] |  |  |
Current regulatory assets | $ 141,744 | $ 215,436 | $ 217,822
Regulatory assets | 309,302 | 322,350 | 351,334
Total regulatory assets | 451,046 | 537,786 | 569,156
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 134,254 | 137,167 | 158,535
Regulatory liabilities | 468,168 | 459,170 | 461,366
Total regulatory liabilities | 602,422 | 596,337 | 619,901
Net regulatory position | $ (151,376) | (58,551) | (50,745)
Environmental compliance programs |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 72,520 | 72,387 | 72,644
Natural gas costs refundable through rate adjustments |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 44,377 | 45,427 | 60,400
Provision for rate refund |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 4,254 | 3,677 | 7,767
Margin sharing |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 3,410 | 4,156 | 2,188
Taxes recoverable from customers |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 2,197 | 2,163 | 2,074
Regulatory liabilities | $ 180,212 | 185,402 | 187,827
Conservation programs |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 1,996 | 2,082 | 3,330
Cost recovery mechanisms |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 636 | 1,720 | 3,841
Regulatory liabilities | 35,794 | 30,354 | 26,286
Other |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 4,864 | 5,555 | 6,291
Regulatory liabilities | 2,344 | 2,161 | 2,024
Plant removal and decommissioning costs |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | 225,142 | 217,603 | 222,668
Accumulated deferred investment tax credit |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | 19,814 | 18,788 | 16,518
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 | $ 63,245 | 76,964 | 84,486
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 | $ 36,567 | 91,091 | 105,064
Regulatory assets | $ 0 | 0 | 15,870
Conservation programs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 21,541 | 19,123 | 15,713
Decoupling mechanisms |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 7,974 | 6,767 | 2,139
Cost recovery mechanisms |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | 6,389 | 5,114 | 6,074
Regulatory assets | $ 70,533 | 76,542 | 82,984
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 |  |
Electric fuel and purchased power deferral |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 2,050 | 9,662 | 3,024
Regulatory assets | 0 | 4,349 | 12,160
Other |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | 3,978 | 6,715 | 1,322
Regulatory assets | $ 4,985 | 5,990 | 6,043
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 | 46,684 | 47,042 | 45,342
Manufactured gas plant site remediation |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | 27,212 | 27,964 | 25,704
Taxes recoverable from customers |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | $ 12,220 | 12,221 | 12,311
Covid-19 deferred costs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 3 years |  |
Regulatory assets | $ 3,886 | 4,167 | 3,245
Long-term debt refinancing costs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 36 years |  |
Regulatory assets | $ 1,718 | $ 2,011 | $ 2,306

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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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