Regulatory assets and liabilities - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024
Regulatory Asset [Abstract]
Current regulatory assets	$ 143,341	$ 215,436	$ 158,614
Regulatory assets	312,008	322,350	414,433
Total regulatory assets	455,349	537,786	573,047
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	140,165	137,167	90,115
Regulatory liabilities	463,879	459,170	523,484
Total regulatory liabilities	604,044	596,337	613,599
Net regulatory position	$ (148,695)	(58,551)	(40,552)
Environmental compliance programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 65,680	72,387	0
Regulatory liabilities	$ 0	0	63,746
Natural gas costs refundable through rate adjustments
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 46,744	45,427	54,282
Demand charges
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 10,000	0	8,645
Provision for rate refund
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 3,833	3,677	6,517
Margin sharing
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 3,562	4,156	2,869
Conservation programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 2,782	2,082	3,576
Taxes recoverable from customers
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 2,087	2,163	1,991
Regulatory liabilities	182,742	185,402	190,850
Cost recovery mechanisms
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	716	1,720	5,642
Regulatory liabilities	33,373	30,354	24,269
Other
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	4,761	5,555	6,593
Regulatory liabilities	2,252	2,161	1,927
Plant removal and decommissioning costs
Regulatory Liability [Abstract]
Regulatory liabilities	221,349	217,603	220,519
Accumulated deferred investment tax credit
Regulatory Liability [Abstract]
Regulatory liabilities	19,301	18,788	16,129
Pension and postretirement benefits
Regulatory Liability [Abstract]
Regulatory liabilities	$ 4,862	4,862	6,044
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	$ 56,441	76,964	7,443
Regulatory assets	$ 0	0	50,372
Natural gas costs recoverable through rate adjustments
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 51,321	91,091	110,481
Regulatory assets	$ 0	0	25,915
Conservation programs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 16,902	19,123	13,735
Electric fuel and purchased power deferral
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 7,109	9,662	17,061
Regulatory assets	0	4,349	15,612
Cost recovery mechanisms
Regulatory Asset [Abstract]
Current regulatory assets	4,806	5,114	6,454
Regulatory assets	$ 72,764	76,542	84,546
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
Decoupling mechanisms
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 4,769	6,767	1,046
Other
Regulatory Asset [Abstract]
Current regulatory assets	1,993	6,715	2,394
Regulatory assets	$ 4,797	5,990	6,053
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	$ 142,065	142,064	142,511
Plant costs/asset retirement obligations
Regulatory Asset [Abstract]
Regulatory assets	46,920	47,042	45,702
Manufactured gas plant site remediation
Regulatory Asset [Abstract]
Regulatory assets	27,443	27,964	25,851
Taxes recoverable from customers
Regulatory Asset [Abstract]
Regulatory assets	$ 12,228	12,221	12,331
Covid-19 deferred costs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	3 years
Regulatory assets	$ 3,926	4,167	3,087
Long-term debt refinancing costs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	36 years
Regulatory assets	$ 1,865	$ 2,011	$ 2,453