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Regulatory assets (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2023
Dec. 31, 2022
Mar. 31, 2022
Regulatory Assets      
Regulatory Assets, Current $ 216,318 $ 165,092 $ 89,078
Regulatory Assets, Noncurrent 349,500 329,659 359,228
Total regulatory assets 565,818 494,751 448,306
Regulatory assets not earning a rate of return $ 255,600 242,500 268,100
Natural gas costs recoverable through rate adjustments      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 Up to 1 year    
Regulatory Assets, Current $ 198,166 141,306 66,132
Conservation programs      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 Up to 1 year    
Regulatory Assets, Current $ 8,599 8,544 7,567
Cost recovery mechanisms      
Regulatory Assets      
Regulatory Assets, Current 3,954 4,019 3,432
Regulatory Assets, Noncurrent $ 65,732 67,171 67,989
Cost recovery mechanisms | Minimum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 Up to 1 year    
Cost recovery mechanisms | Maximum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 Up to 10 years    
Other regulatory assets      
Regulatory Assets      
Regulatory Assets, Current $ 5,599 11,223 11,947
Regulatory Assets, Noncurrent $ 11,430 11,010 13,564
Other regulatory assets | Minimum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 Up to 1 year    
Other regulatory assets | Maximum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 Up to 16 years    
Pension and postretirement benefits      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 **    
Regulatory Assets, Noncurrent $ 143,349 143,349 142,681
Plant costs/asset retirement obligations      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 Over plant lives    
Regulatory Assets, Noncurrent $ 44,131 44,462 63,325
Manufactured gas plant site remediation      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 -    
Regulatory Assets, Noncurrent $ 26,605 26,624 26,089
Environmental compliance programs      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 -    
Regulatory Assets, Noncurrent $ 21,942 0 0
Plant to be retired      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 -    
Regulatory Assets, Noncurrent $ 21,072 21,525 29,452
Taxes recoverable from customers      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 Over plant lives    
Regulatory Assets, Noncurrent $ 12,198 12,330 12,492
Long-term debt refinancing costs      
Regulatory Assets      
Estimated Recovery or Refund Period as of March 31, 2023 Up to 37 years    
Regulatory Assets, Noncurrent $ 3,041 $ 3,188 $ 3,636