Regulatory assets and liabilities - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Regulatory Asset [Abstract]
Current regulatory assets	$ 217,822	$ 172,492	$ 193,162
Regulatory assets	351,334	447,099	345,185
Total regulatory assets	569,156	619,591	538,347
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	158,535	70,761	48,057
Regulatory liabilities	461,366	521,050	455,877
Total regulatory liabilities	619,901	591,811	503,934
Net regulatory position	$ (50,745)	27,780	34,413
Environmental compliance programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 72,644	0	0
Regulatory liabilities	$ 0	61,941	0
Natural gas costs refundable through rate adjustments
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 60,400	43,161	17,820
Provision for rate refund
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 7,767	6,866	1,227
Cost recovery mechanisms
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	3,841	6,284	3,304
Regulatory liabilities	$ 26,286	21,791	18,226
Conservation programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 3,330	2,130	2,851
Margin sharing
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 2,188	5,243	1,791
Taxes recoverable from customers
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 2,074	2,149	1,513
Regulatory liabilities	$ 187,827	193,578	197,757
Refundable fuel and electric costs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 1,431	263	103
Electric fuel and purchased power deferral
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 506	0	8,481
Other
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	4,354	4,665	10,967
Regulatory liabilities	2,024	1,809	1,694
Plant removal and decommissioning costs
Regulatory Liability [Abstract]
Regulatory liabilities	222,668	220,147	216,682
Accumulated deferred investment tax credit
Regulatory Liability [Abstract]
Regulatory liabilities	16,518	15,740	14,398
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	$ 105,064	98,844	170,249
Regulatory assets	$ 15,870	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
Environmental compliance programs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 84,486	5,525	2,926
Regulatory assets	$ 2,858	66,806	20,611
Conservation programs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 15,713	14,425	10,897
Cost recovery mechanisms
Regulatory Asset [Abstract]
Current regulatory assets	6,074	9,153	4,758
Regulatory assets	$ 82,984	85,944	63,890
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
Electric fuel and purchased power deferral
Regulatory Asset [Abstract]
Current regulatory assets	$ 3,024	33,918	2,221
Regulatory assets	$ 12,160	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
Other
Regulatory Asset [Abstract]
Current regulatory assets	$ 3,461	10,627	2,111
Regulatory assets	$ 9,061	8,588	10,766
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	45,342	46,009	43,855
Manufactured gas plant site remediation
Regulatory Asset [Abstract]
Regulatory assets	25,704	26,127	25,764
Taxes recoverable from customers
Regulatory Asset [Abstract]
Regulatory assets	$ 12,311	12,249	12,099
Long-term debt refinancing costs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	36 years
Regulatory assets	$ 2,306	2,600	2,894
Plant removal and decommissioning costs
Regulatory Asset [Abstract]
Regulatory assets	$ 227	$ 772	$ 20,858