v3.26.1
Regulatory matters - Schedule of Unamortized Regulatory Assets and Liabilities (Details) - USD ($)
$ in Thousands
Jun. 30, 2026
Dec. 31, 2025
Jun. 30, 2025
Regulatory assets:      
Current regulatory assets $ 192,794 $ 179,579 $ 141,744
Regulatory assets 295,865 297,218 309,302
Total regulatory assets 488,659 476,797 451,046
Regulatory liabilities:      
Regulatory liabilities due within one year 156,951 148,584 134,254
Regulatory liabilities 478,405 472,329 468,168
Total regulatory liabilities 635,356 620,913 602,422
Net regulatory position $ (146,697) (144,116) (151,376)
Environmental compliance programs      
Regulatory liabilities:      
Estimated Recovery or Refund Period 1 year    
Regulatory liabilities due within one year $ 100,186 89,306 72,520
Natural gas costs refundable through rate adjustments      
Regulatory liabilities:      
Estimated Recovery or Refund Period 1 year    
Regulatory liabilities due within one year $ 34,598 47,130 44,377
Electric fuel and purchased power deferral      
Regulatory liabilities:      
Estimated Recovery or Refund Period 1 year    
Regulatory liabilities due within one year $ 10,465 0 934
Margin sharing      
Regulatory liabilities:      
Estimated Recovery or Refund Period 1 year    
Regulatory liabilities due within one year $ 4,065 3,946 3,410
Plant removal and decommissioning costs      
Regulatory liabilities:      
Regulatory liabilities $ 230,673 224,313 225,142
Taxes recoverable from customers      
Regulatory liabilities:      
Estimated Recovery or Refund Period 1 year    
Regulatory liabilities due within one year $ 2,313 2,301 2,197
Regulatory liabilities $ 171,179 176,665 180,212
Provision for rate refund      
Regulatory liabilities:      
Estimated Recovery or Refund Period 1 year    
Regulatory liabilities due within one year $ 1,353 1,780 4,254
Conservation programs      
Regulatory liabilities:      
Estimated Recovery or Refund Period 1 year    
Regulatory liabilities due within one year $ 851 733 1,996
Cost recovery mechanisms      
Regulatory liabilities:      
Regulatory liabilities 46,098 41,323 35,794
Accumulated deferred investment tax credit      
Regulatory liabilities:      
Regulatory liabilities 22,831 22,663 19,814
Pension and postretirement benefits      
Regulatory liabilities:      
Regulatory liabilities 4,776 4,776 4,862
Other      
Regulatory liabilities:      
Regulatory liabilities due within one year 3,120 3,388 4,566
Regulatory liabilities $ 2,848 2,589 2,344
Minimum | Other      
Regulatory liabilities:      
Estimated Recovery or Refund Period 1 year    
Maximum | Cost recovery mechanisms      
Regulatory liabilities:      
Estimated Recovery or Refund Period 16 years    
Maximum | Other      
Regulatory liabilities:      
Estimated Recovery or Refund Period 12 years    
Environmental compliance programs      
Regulatory assets:      
Estimated Recovery or Refund Period 1 year    
Current regulatory assets $ 95,915 78,784 63,245
Conservation programs      
Regulatory assets:      
Estimated Recovery or Refund Period 1 year    
Current regulatory assets $ 33,426 29,148 21,541
Decoupling mechanisms      
Regulatory assets:      
Estimated Recovery or Refund Period 1 year    
Current regulatory assets $ 23,272 17,091 7,974
Pension and postretirement benefits      
Regulatory assets:      
Regulatory assets 135,222 135,222 142,064
Cost recovery mechanisms      
Regulatory assets:      
Current regulatory assets 18,376 13,460 6,389
Regulatory assets $ 58,977 63,328 70,533
Cost recovery mechanisms | Minimum      
Regulatory assets:      
Estimated Recovery or Refund Period 1 year    
Cost recovery mechanisms | Maximum      
Regulatory assets:      
Estimated Recovery or Refund Period 23 years    
Natural gas costs recoverable through rate adjustments      
Regulatory assets:      
Estimated Recovery or Refund Period 1 year    
Current regulatory assets $ 11,334 22,897 36,567
Electric fuel and purchased power deferral      
Regulatory assets:      
Estimated Recovery or Refund Period 1 year    
Current regulatory assets $ 2,043 6,902 2,050
Plant costs/asset retirement obligations      
Regulatory assets:      
Regulatory assets 48,088 48,352 46,684
Manufactured gas plant site remediation      
Regulatory assets:      
Regulatory assets 28,042 28,411 27,212
Taxes recoverable from customers      
Regulatory assets:      
Regulatory assets $ 12,101 12,250 12,220
Covid-19 deferred costs      
Regulatory assets:      
Estimated Recovery or Refund Period 2 years    
Regulatory assets $ 3,590 3,761 3,886
Long-term debt refinancing costs      
Regulatory assets:      
Estimated Recovery or Refund Period 37 years    
Regulatory assets $ 1,500 1,799 1,718
Other      
Regulatory assets:      
Current regulatory assets 8,428 11,297 3,978
Regulatory assets $ 8,345 $ 4,095 $ 4,985
Other | Minimum      
Regulatory assets:      
Estimated Recovery or Refund Period 1 year    
Other | Maximum      
Regulatory assets:      
Estimated Recovery or Refund Period 13 years