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Regulatory assets (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2021
Feb. 28, 2021
Dec. 31, 2020
Jun. 30, 2020
Regulatory Assets        
Regulatory Assets, Current $ 109,581   $ 68,527 $ 60,557
Regulatory Assets, Noncurrent 376,913   379,381 351,025
Total regulatory assets 486,494   447,908 411,582
Regulatory assets not earning a rate of return 317,000   332,500 296,600
Natural gas costs recoverable through rate adjustments        
Regulatory Assets        
Regulatory Assets, Current 85,045   42,481 42,582
Regulatory Assets, Noncurrent $ 8,389   21,539 27,184
Natural gas costs recoverable through rate adjustments | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Up to 1 year      
Natural gas costs recoverable through rate adjustments | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Up to 3 years      
Conservation programs        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Up to 1 year      
Regulatory Assets, Current $ 7,580   7,117 7,256
Cost recovery mechanisms        
Regulatory Assets        
Regulatory Assets, Current 5,692   10,645 7,781
Regulatory Assets, Noncurrent $ 44,415   16,245 13,140
Cost recovery mechanisms | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Up to 1 year      
Cost recovery mechanisms | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Up to 10 years      
Pension and postretirement benefits        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 **      
Regulatory Assets, Noncurrent $ 155,906   155,942 157,033
Plant costs/asset retirement obligations        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Over plant lives      
Regulatory Assets, Noncurrent $ 72,853   71,740 68,191
Plant retirement        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 -      
Regulatory Assets, Noncurrent $ 46,061   65,919 49,185
Manufactured gas plant site remediation        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 -      
Regulatory Assets, Noncurrent $ 26,155   26,429 14,826
Taxes recoverable from/refundable to customers        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Over plant lives      
Regulatory Assets, Noncurrent $ 10,929   10,785 10,890
Long-term debt refinancing costs        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Up to 39 years      
Regulatory Assets, Noncurrent $ 4,110   4,426 3,980
Other regulatory assets/liabilities        
Regulatory Assets        
Regulatory Assets, Current 11,264   8,284 2,938
Regulatory Assets, Noncurrent $ 8,095   $ 6,356 $ 6,596
Other regulatory assets/liabilities | Minimum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Up to 1 year      
Other regulatory assets/liabilities | Maximum [Member]        
Regulatory Assets        
Estimated Recovery or Refund Period as of June 30, 2021 Up to 18 years      
Increased Gas Costs        
Regulatory Assets        
Total regulatory assets   $ 44,000