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Regulatory assets (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2022
Dec. 31, 2021
Jun. 30, 2021
Feb. 28, 2021
Regulatory Assets        
Regulatory Assets, Current $ 102,535 $ 118,691 $ 109,581  
Regulatory Assets, Noncurrent 347,061 357,851 376,913  
Total regulatory assets 449,596 476,542 486,494  
Regulatory assets not earning a rate of return 262,900 296,600 317,000  
Natural gas costs recoverable through rate adjustments        
Regulatory Assets        
Regulatory Assets, Current 83,557 86,371 85,045  
Regulatory Assets, Noncurrent $ 1,839 5,186 8,389  
Total regulatory assets       $ 44,000
Natural gas costs recoverable through rate adjustments | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Up to 1 year      
Natural gas costs recoverable through rate adjustments | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Up to 2 years      
Conservation programs        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Up to 1 year      
Regulatory Assets, Current $ 7,883 8,225 7,580  
Decoupling        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Up to 1 year      
Regulatory Assets, Current $ 4,615 9,131 6,750  
Cost recovery mechanisms        
Regulatory Assets        
Regulatory Assets, Current 3,459 4,536 5,692  
Regulatory Assets, Noncurrent $ 64,096 44,870 44,415  
Cost recovery mechanisms | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Up to 1 year      
Cost recovery mechanisms | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Up to 10 years      
Other regulatory assets        
Regulatory Assets        
Regulatory Assets, Current $ 3,021 10,428 4,514  
Regulatory Assets, Noncurrent $ 10,704 9,742 8,095  
Other regulatory assets | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Up to 1 year      
Other regulatory assets | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Up to 17 years      
Pension and postretirement benefits        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 **      
Regulatory Assets, Noncurrent $ 137,582 142,681 155,906  
Plant costs/asset retirement obligations        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Over plant lives      
Regulatory Assets, Noncurrent $ 63,334 63,116 72,853  
Plant retirement        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 -      
Regulatory Assets, Noncurrent $ 27,628 50,070 46,061  
Manufactured gas plant site remediation        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 -      
Regulatory Assets, Noncurrent $ 26,031 26,053 26,155  
Taxes recoverable from customers        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Over plant lives      
Regulatory Assets, Noncurrent $ 12,365 12,339 10,929  
Long-term debt refinancing costs        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2022 Up to 38 years      
Regulatory Assets, Noncurrent $ 3,482 $ 3,794 $ 4,110