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Regulatory assets (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2023
Dec. 31, 2022
Sep. 30, 2022
Regulatory Assets      
Regulatory Assets, Current $ 169,248 $ 165,092 $ 148,160
Regulatory Assets, Noncurrent 428,969 329,659 344,442
Total regulatory assets 598,217 494,751 492,602
Regulatory assets not earning a rate of return $ 211,800 242,500 264,400
Natural gas costs recoverable through rate adjustments      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Up to 1 year    
Regulatory Assets, Current $ 107,101 141,306 112,079
Regulatory Assets, Noncurrent $ 64,914 0 462
Natural gas costs recoverable through rate adjustments | Maximum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Up to 2 years    
Conservation programs      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Up to 1 year    
Regulatory Assets, Current $ 14,411 8,544 9,363
Cost recovery mechanisms      
Regulatory Assets      
Regulatory Assets, Current 9,461 4,019 3,354
Regulatory Assets, Noncurrent $ 68,539 67,171 67,094
Cost recovery mechanisms | Minimum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Up to 1 year    
Cost recovery mechanisms | Maximum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Up to 10 years    
Other regulatory assets      
Regulatory Assets      
Regulatory Assets, Current $ 20,500 8,567 21,036
Regulatory Assets, Noncurrent $ 11,406 11,010 10,931
Other regulatory assets | Minimum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Up to 1 year    
Other regulatory assets | Maximum [Member]      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Up to 16 years    
Pension and postretirement benefits      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 **    
Regulatory Assets, Noncurrent $ 144,448 143,349 137,582
Plant costs/asset retirement obligations      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Over plant lives    
Regulatory Assets, Noncurrent $ 43,520 44,462 61,941
Manufactured gas plant site remediation      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 -    
Regulatory Assets, Noncurrent $ 24,577 26,624 25,963
Plant to be retired      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 -    
Regulatory Assets, Noncurrent $ 19,947 21,525 24,740
Environmental compliance programs      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 -    
Regulatory Assets, Noncurrent $ 36,605 0 0
Taxes recoverable from customers      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Over plant lives    
Regulatory Assets, Noncurrent $ 12,266 12,330 12,394
Long-term debt refinancing costs      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Up to 37 years    
Regulatory Assets, Noncurrent $ 2,747 3,188 3,335
Regulatory Asset, Electric Fuel & Purchased Power Deferral      
Regulatory Assets      
Estimated Recovery or Refund Period as of September 30, 2023 Up to 1 year    
Regulatory Assets, Current $ 17,775 $ 2,656 $ 2,328