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Regulatory assets (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2022
Dec. 31, 2021
Mar. 31, 2021
Feb. 28, 2021
Regulatory Assets        
Regulatory Assets, Current $ 89,078 $ 118,691 $ 98,452  
Regulatory Assets, Noncurrent 359,228 357,851 379,917  
Total regulatory assets 448,306 476,542 478,369  
Regulatory assets not earning a rate of return 268,100 296,600 361,400  
Natural gas costs recoverable through rate adjustments        
Regulatory Assets        
Regulatory Assets, Current 66,132 86,371 76,782  
Regulatory Assets, Noncurrent $ 3,246 5,186 15,158  
Total regulatory assets       $ 44,000
Natural gas costs recoverable through rate adjustments | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Up to 1 year      
Natural gas costs recoverable through rate adjustments | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Up to 2 years      
Decoupling        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Up to 1 year      
Regulatory Assets, Current $ 7,609 9,131 4,180  
Conservation programs        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Up to 1 year      
Regulatory Assets, Current $ 7,567 8,225 6,679  
Cost recovery mechanisms        
Regulatory Assets        
Regulatory Assets, Current 3,432 4,536 7,130  
Regulatory Assets, Noncurrent $ 67,989 44,870 15,508  
Cost recovery mechanisms | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Up to 1 year      
Cost recovery mechanisms | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Up to 10 years      
Other regulatory assets        
Regulatory Assets        
Regulatory Assets, Current $ 4,338 10,428 3,681  
Regulatory Assets, Noncurrent $ 10,318 9,742 6,509  
Other regulatory assets | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Up to 1 year      
Other regulatory assets | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Up to 17 years      
Pension and postretirement benefits        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 **      
Regulatory Assets, Noncurrent $ 142,681 142,681 155,924  
Plant costs/asset retirement obligations        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Over plant lives      
Regulatory Assets, Noncurrent $ 63,325 63,116 72,250  
Plant retirement        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 -      
Regulatory Assets, Noncurrent $ 29,452 50,070 73,498  
Manufactured gas plant site remediation        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 -      
Regulatory Assets, Noncurrent $ 26,089 26,053 26,002  
Taxes recoverable from customers        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Over plant lives      
Regulatory Assets, Noncurrent $ 12,492 12,339 10,800  
Long-term debt refinancing costs        
Regulatory Assets        
Estimated Recovery or Refund Period as of March 31, 2022 Up to 38 years      
Regulatory Assets, Noncurrent $ 3,636 $ 3,794 $ 4,268