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Regulatory assets (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2023
Dec. 31, 2022
Jun. 30, 2022
Regulatory Assets      
Regulatory Assets, Current $ 193,162 $ 165,092 $ 102,535
Regulatory Assets, Noncurrent 345,185 329,659 347,061
Total regulatory assets 538,347 494,751 449,596
Regulatory assets not earning a rate of return $ 226,400 242,500 262,900
Natural gas costs recoverable through rate adjustments      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 Up to 1 year    
Regulatory Assets, Current $ 170,249 141,306 83,557
Conservation programs      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 Up to 1 year    
Regulatory Assets, Current $ 10,897 8,544 7,883
Cost recovery mechanisms      
Regulatory Assets      
Regulatory Assets, Current 4,758 4,019 3,459
Regulatory Assets, Noncurrent $ 63,890 67,171 64,096
Cost recovery mechanisms | Minimum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 Up to 1 year    
Cost recovery mechanisms | Maximum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 Up to 10 years    
Other regulatory assets      
Regulatory Assets      
Regulatory Assets, Current $ 7,258 11,223 7,636
Regulatory Assets, Noncurrent $ 10,766 11,010 12,543
Other regulatory assets | Minimum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 Up to 1 year    
Other regulatory assets | Maximum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 Up to 16 years    
Pension and postretirement benefits      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 **    
Regulatory Assets, Noncurrent $ 144,448 143,349 137,582
Plant costs/asset retirement obligations      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 Over plant lives    
Regulatory Assets, Noncurrent $ 43,855 44,462 63,334
Manufactured gas plant site remediation      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 -    
Regulatory Assets, Noncurrent $ 25,764 26,624 26,031
Plant to be retired      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 -    
Regulatory Assets, Noncurrent $ 20,858 21,525 27,628
Environmental compliance programs      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 -    
Regulatory Assets, Noncurrent $ 20,611 0 0
Taxes recoverable from customers      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 Over plant lives    
Regulatory Assets, Noncurrent $ 12,099 12,330 12,365
Long-term debt refinancing costs      
Regulatory Assets      
Estimated Recovery or Refund Period as of June 30, 2023 Up to 37 years    
Regulatory Assets, Noncurrent $ 2,894 $ 3,188 $ 3,482