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Regulatory assets and liabilities - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023
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Regulatory Asset [Abstract] |  |  |
Current regulatory assets | $ 158,614 | $ 172,492 | $ 216,318
Regulatory assets | 414,433 | 447,099 | 349,500
Total regulatory assets | 573,047 | 619,591 | 565,818
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 90,115 | 70,761 | 44,395
Regulatory liabilities | 523,484 | 521,050 | 454,669
Total regulatory liabilities | 613,599 | 591,811 | 499,064
Net regulatory position | $ (40,552) | 27,780 | 66,754
Natural gas costs refundable through rate adjustments |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 54,282 | 43,161 | 6,331
Provision for rate refund |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 6,517 | 6,866 | 1,115
Cost recovery mechanisms |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 5,642 | 6,284 | 2,926
Regulatory liabilities | $ 24,269 | 21,791 | 16,313
Conservation programs |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 3,576 | 2,130 | 3,834
Margin sharing |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 2,869 | 5,243 | 0
Taxes recoverable from customers |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 1,991 | 2,149 | 1,513
Regulatory liabilities | $ 190,850 | 193,578 | 200,879
Refundable fuel and electric costs |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 1,122 | 263 | 1,179
Electric fuel and purchased power deferral |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 746 | 0 | 9,630
Other |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 13,370 | 4,665 | 17,867
Regulatory liabilities | 1,927 | 1,809 | 1,638
Plant to be retired |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | 220,519 | 220,147 | 214,467
Environmental compliance programs |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | 63,746 | 61,941 | 0
Accumulated deferred investment tax credit |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | 16,129 | 15,740 | 13,996
Pension and postretirement benefits |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | $ 6,044 | 6,044 | 7,376
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 | 18 years |  |
Maximum | Other |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 14 years |  |
Natural gas costs recoverable through rate adjustments |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | $ 110,481 | 98,844 | 198,166
Regulatory assets | $ 25,915 | 55,493 | 0
Natural gas costs recoverable through rate adjustments | Minimum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Natural gas costs recoverable through rate adjustments | Maximum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 2 years |  |
Electric fuel and purchased power deferral |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | $ 17,061 | 33,918 | 3,279
Regulatory assets | $ 15,612 | 0 | 0
Electric fuel and purchased power deferral | Minimum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Electric fuel and purchased power deferral | Maximum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 2 years |  |
Conservation programs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 13,735 | 14,425 | 8,599
Environmental compliance programs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 7,443 | 5,525 | 1,191
Regulatory assets | 50,372 | 66,806 | 21,942
Cost recovery mechanisms |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | 6,454 | 9,153 | 3,954
Regulatory assets | $ 84,546 | 85,944 | 65,732
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 | 25 years |  |
Other |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | $ 3,440 | 10,627 | 1,129
Regulatory assets | $ 8,846 | 8,588 | 11,430
Other | Minimum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Other | Maximum |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 15 years |  |
Pension and postretirement benefits |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | $ 142,511 | 142,511 | 143,349
Plant costs/asset retirement obligations |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | 45,702 | 46,009 | 44,131
Manufactured gas plant site remediation |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | 25,851 | 26,127 | 26,605
Taxes recoverable from customers |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | $ 12,331 | 12,249 | 12,198
Long-term debt refinancing costs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 36 years |  |
Regulatory assets | $ 2,453 | 2,600 | 3,041
Plant to be retired |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | $ 294 | $ 772 | $ 21,072

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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//1943274/2147481878/980-340-50-1 + Details
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