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Regulatory assets and liabilities - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023 | Sep. 30, 2023
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Regulatory Asset [Abstract] |  |  |
Current regulatory assets | $ 241,008 | $ 172,492 | $ 169,248
Regulatory assets | 343,578 | 447,099 | 428,969
Total regulatory assets | 584,586 | 619,591 | 598,217
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 157,068 | 70,761 | 45,013
Regulatory liabilities | 463,790 | 521,050 | 495,112
Total regulatory liabilities | 620,858 | 591,811 | 540,125
Net regulatory position | $ (36,272) | 27,780 | 58,092
Environmental compliance programs |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 77,392 | 0 | 0
Regulatory liabilities | $ 0 | 61,941 | 36,126
Natural gas costs refundable through rate adjustments |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 54,197 | 43,161 | 20,445
Provision for rate refund |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 8,868 | 6,866 | 2,717
Margin sharing |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 3,945 | 5,243 | 2,697
Conservation programs |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 2,662 | 2,130 | 1,976
Cost recovery mechanisms |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 2,634 | 6,284 | 5,754
Regulatory liabilities | $ 28,397 | 21,791 | 20,012
Taxes recoverable from customers |  |  |
Regulatory Liability [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Regulatory liabilities due within one year | $ 2,195 | 2,149 | 1,513
Regulatory liabilities | 184,715 | 193,578 | 194,804
Other |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities due within one year | 5,175 | 4,928 | 9,911
Regulatory liabilities | 2,117 | 1,809 | 1,751
Plant removal and decommissioning costs |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | 224,839 | 220,147 | 220,499
Accumulated deferred investment tax credit |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | 17,679 | 15,740 | 14,800
Pension and postretirement benefits |  |  |
Regulatory Liability [Abstract] |  |  |
Regulatory liabilities | $ 6,043 | 6,044 | 7,120
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 | $ 111,946 | 98,844 | 107,101
Regulatory assets | $ 11,707 | 55,493 | 64,914
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 |  |
Environmental compliance programs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 80,198 | 5,525 | 3,972
Regulatory assets | $ 2,858 | 66,806 | 36,605
Conservation programs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 19,059 | 14,425 | 14,411
Demand cost deferral |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 1 year |  |
Current regulatory assets | $ 13,898 | 5,207 | 13,254
Electric fuel and purchased power deferral |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | 4,497 | 33,918 | 17,775
Regulatory assets | $ 8,335 | 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 |  |
Cost recovery mechanisms |  |  |
Regulatory Asset [Abstract] |  |  |
Current regulatory assets | $ 4,332 | 9,153 | 9,461
Regulatory assets | $ 82,907 | 85,944 | 68,539
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 | $ 7,078 | 5,420 | 3,274
Regulatory assets | $ 9,725 | 8,588 | 11,406
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 | 144,448
Plant costs/asset retirement obligations |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | 45,249 | 46,009 | 43,520
Manufactured gas plant site remediation |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | 25,789 | 26,127 | 24,577
Taxes recoverable from customers |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | $ 12,316 | 12,249 | 12,266
Long-term debt refinancing costs |  |  |
Regulatory Asset [Abstract] |  |  |
Estimated Recovery or Refund Period | 36 years |  |
Regulatory assets | $ 2,159 | 2,600 | 2,747
Plant removal and decommissioning costs |  |  |
Regulatory Asset [Abstract] |  |  |
Regulatory assets | $ 22 | $ 772 | $ 19,947

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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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