Regulatory assets and liabilities - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024
Regulatory Asset [Abstract]
Current regulatory assets	$ 179,149	$ 215,436	$ 241,008
Regulatory assets	305,890	322,350	343,578
Total regulatory assets	485,039	537,786	584,586
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	143,822	137,167	157,068
Regulatory liabilities	473,812	459,170	463,790
Total regulatory liabilities	617,634	596,337	620,858
Net regulatory position	$ (132,595)	(58,551)	(36,272)
Environmental compliance programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 84,884	72,387	77,392
Natural gas costs refundable through rate adjustments
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 43,373	45,427	54,197
Provision for rate refund
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 4,358	3,677	8,868
Margin sharing
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 3,831	4,156	3,945
Taxes recoverable from customers
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 2,325	2,163	2,195
Regulatory liabilities	$ 177,469	185,402	184,715
Conservation programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 1,108	2,082	2,662
Cost recovery mechanisms
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	154	1,720	2,634
Regulatory liabilities	38,426	30,354	28,397
Other
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	3,789	5,555	5,175
Regulatory liabilities	2,458	2,161	2,117
Plant removal and decommissioning costs
Regulatory Liability [Abstract]
Regulatory liabilities	228,982	217,603	224,839
Accumulated deferred investment tax credit
Regulatory Liability [Abstract]
Regulatory liabilities	21,615	18,788	17,679
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	$ 69,646	76,964	80,198
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	$ 34,325	91,091	111,946
Regulatory assets	$ 0	0	11,707
Conservation programs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 27,405	19,123	19,059
Demand cost deferral
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 15,287	5,237	13,898
Decoupling mechanisms
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 9,716	6,767	4,795
Electric fuel and purchased power deferral
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 9,644	9,662	4,497
Regulatory assets	0	4,349	8,335
Cost recovery mechanisms
Regulatory Asset [Abstract]
Current regulatory assets	8,606	5,114	4,332
Regulatory assets	$ 68,314	76,542	82,907
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
Other
Regulatory Asset [Abstract]
Current regulatory assets	$ 4,520	1,478	2,283
Regulatory assets	$ 4,340	5,990	5,890
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	46,334	47,042	45,249
Manufactured gas plant site remediation
Regulatory Asset [Abstract]
Regulatory assets	27,147	27,964	25,789
Taxes recoverable from customers
Regulatory Asset [Abstract]
Regulatory assets	$ 12,257	12,221	12,316
Covid-19 deferred costs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	3 years
Regulatory assets	$ 3,863	4,167	3,857
Long-term debt refinancing costs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	36 years
Regulatory assets	$ 1,571	$ 2,011	$ 2,159