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Regulatory assets (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2021
Feb. 28, 2021
Dec. 31, 2020
Sep. 30, 2020
Regulatory Assets        
Regulatory Assets, Current $ 133,135   $ 68,527 $ 79,125
Regulatory Assets, Noncurrent 379,245   379,381 369,764
Total regulatory assets 512,380   447,908 448,889
Regulatory assets not earning a rate of return 318,200   332,500 318,200
Natural gas costs recoverable through rate adjustments        
Regulatory Assets        
Regulatory Assets, Current 98,211   42,481 48,478
Regulatory Assets, Noncurrent $ 6,084   21,539 24,677
Total regulatory assets   $ 44,000    
Natural gas costs recoverable through rate adjustments | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Up to 1 year      
Natural gas costs recoverable through rate adjustments | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Up to 3 years      
Conservation programs        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Up to 1 year      
Regulatory Assets, Current $ 8,495   7,117 7,857
Cost recovery mechanisms        
Regulatory Assets        
Regulatory Assets, Current 4,693   10,645 8,177
Regulatory Assets, Noncurrent $ 43,955   16,245 14,281
Cost recovery mechanisms | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Up to 1 year      
Cost recovery mechanisms | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Up to 10 years      
Other regulatory assets        
Regulatory Assets        
Regulatory Assets, Current $ 21,736   8,284 14,613
Regulatory Assets, Noncurrent $ 9,856   6,356 6,840
Other regulatory assets | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Up to 1 year      
Other regulatory assets | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Up to 18 years      
Pension and postretirement benefits        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 **      
Regulatory Assets, Noncurrent $ 155,888   155,942 157,015
Plant costs/asset retirement obligations        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Over plant lives      
Regulatory Assets, Noncurrent $ 73,528   71,740 68,815
Plant retirement        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 -      
Regulatory Assets, Noncurrent $ 48,544   65,919 57,499
Manufactured gas plant site remediation        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 -      
Regulatory Assets, Noncurrent $ 26,000   26,429 25,964
Taxes recoverable from customers        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Over plant lives      
Regulatory Assets, Noncurrent $ 11,438   10,785 10,847
Long-term debt refinancing costs        
Regulatory Assets        
Estimated Recovery or Refund Period as of September 30, 2021 Up to 39 years      
Regulatory Assets, Noncurrent $ 3,952   $ 4,426 $ 3,826