Regulatory assets and liabilities - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Regulatory Asset [Abstract]
Current regulatory assets	$ 241,008	$ 172,492	$ 169,248
Regulatory assets	343,578	447,099	428,969
Total regulatory assets	584,586	619,591	598,217
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	157,068	70,761	45,013
Regulatory liabilities	463,790	521,050	495,112
Total regulatory liabilities	620,858	591,811	540,125
Net regulatory position	$ (36,272)	27,780	58,092
Environmental compliance programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 77,392	0	0
Regulatory liabilities	$ 0	61,941	36,126
Natural gas costs refundable through rate adjustments
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 54,197	43,161	20,445
Provision for rate refund
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 8,868	6,866	2,717
Margin sharing
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 3,945	5,243	2,697
Conservation programs
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 2,662	2,130	1,976
Cost recovery mechanisms
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	2,634	6,284	5,754
Regulatory liabilities	$ 28,397	21,791	20,012
Taxes recoverable from customers
Regulatory Liability [Abstract]
Estimated Recovery or Refund Period	1 year
Regulatory liabilities due within one year	$ 2,195	2,149	1,513
Regulatory liabilities	184,715	193,578	194,804
Other
Regulatory Liability [Abstract]
Regulatory liabilities due within one year	5,175	4,928	9,911
Regulatory liabilities	2,117	1,809	1,751
Plant removal and decommissioning costs
Regulatory Liability [Abstract]
Regulatory liabilities	224,839	220,147	220,499
Accumulated deferred investment tax credit
Regulatory Liability [Abstract]
Regulatory liabilities	17,679	15,740	14,800
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	$ 111,946	98,844	107,101
Regulatory assets	$ 11,707	55,493	64,914
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	$ 80,198	5,525	3,972
Regulatory assets	$ 2,858	66,806	36,605
Conservation programs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 19,059	14,425	14,411
Demand cost deferral
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	1 year
Current regulatory assets	$ 13,898	5,207	13,254
Electric fuel and purchased power deferral
Regulatory Asset [Abstract]
Current regulatory assets	4,497	33,918	17,775
Regulatory assets	$ 8,335	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
Cost recovery mechanisms
Regulatory Asset [Abstract]
Current regulatory assets	$ 4,332	9,153	9,461
Regulatory assets	$ 82,907	85,944	68,539
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	$ 7,078	5,420	3,274
Regulatory assets	$ 9,725	8,588	11,406
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	45,249	46,009	43,520
Manufactured gas plant site remediation
Regulatory Asset [Abstract]
Regulatory assets	25,789	26,127	24,577
Taxes recoverable from customers
Regulatory Asset [Abstract]
Regulatory assets	$ 12,316	12,249	12,266
Long-term debt refinancing costs
Regulatory Asset [Abstract]
Estimated Recovery or Refund Period	36 years
Regulatory assets	$ 2,159	2,600	2,747
Plant removal and decommissioning costs
Regulatory Asset [Abstract]
Regulatory assets	$ 22	$ 772	$ 19,947