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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
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

The net amount of all regulatory assets less all regulatory liabilities as of the end of the period.

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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.

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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.

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