Regulatory assets and liabilities - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024
Regulatory Asset [Abstract]
Current regulatory assets	$ 141,744	$ 215,436	$ 217,822
Regulatory assets	309,302	322,350	351,334
Total regulatory assets	451,046	537,786	569,156
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	134,254	137,167	158,535
Regulatory liabilities	468,168	459,170	461,366
Total regulatory liabilities	602,422	596,337	619,901
Net regulatory position	$ (151,376)	(58,551)	(50,745)
Environmental compliance programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 72,520	72,387	72,644
Natural gas costs refundable through rate adjustments
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 44,377	45,427	60,400
Provision for rate refund
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 4,254	3,677	7,767
Margin sharing
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 3,410	4,156	2,188
Taxes recoverable from customers
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 2,197	2,163	2,074
Regulatory liabilities	$ 180,212	185,402	187,827
Conservation programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 1,996	2,082	3,330
Cost recovery mechanisms
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	636	1,720	3,841
Regulatory liabilities	35,794	30,354	26,286
Other
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	4,864	5,555	6,291
Regulatory liabilities	2,344	2,161	2,024
Plant removal and decommissioning costs
Regulatory Liability [Abstract]
Regulatory liabilities	225,142	217,603	222,668
Accumulated deferred investment tax credit
Regulatory Liability [Abstract]
Regulatory liabilities	19,814	18,788	16,518
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	$ 63,245	76,964	84,486
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	$ 36,567	91,091	105,064
Regulatory assets	$ 0	0	15,870
Conservation programs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 21,541	19,123	15,713
Decoupling mechanisms
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 7,974	6,767	2,139
Cost recovery mechanisms
Regulatory Asset [Abstract]
Current regulatory assets	6,389	5,114	6,074
Regulatory assets	$ 70,533	76,542	82,984
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
Electric fuel and purchased power deferral
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 2,050	9,662	3,024
Regulatory assets	0	4,349	12,160
Other
Regulatory Asset [Abstract]
Current regulatory assets	3,978	6,715	1,322
Regulatory assets	$ 4,985	5,990	6,043
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	46,684	47,042	45,342
Manufactured gas plant site remediation
Regulatory Asset [Abstract]
Regulatory assets	27,212	27,964	25,704
Taxes recoverable from customers
Regulatory Asset [Abstract]
Regulatory assets	$ 12,220	12,221	12,311
Covid-19 deferred costs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	3 years
Regulatory assets	$ 3,886	4,167	3,245
Long-term debt refinancing costs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	36 years
Regulatory assets	$ 1,718	$ 2,011	$ 2,306