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Regulatory assets and liabilities Regulatory Assets (Details 1) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Sep. 30, 2019
Regulatory Assets [Line Items]      
Regulatory Assets, Current $ 79,125 $ 63,613 $ 69,181
Regulatory Assets, Noncurrent 369,764 353,784 362,799
Total regulatory assets 448,889 417,397 431,980
Regulatory assets not earning a rate of return 318,200 276,500 278,600
Natural gas costs recoverable through rate adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets, Current 48,478 42,823 39,383
Regulatory Assets, Noncurrent 24,677 46,381 53,618
Cost recovery mechanisms      
Regulatory Assets [Line Items]      
Regulatory Assets, Current 8,177 6,288 8,706
Regulatory Assets, Noncurrent $ 14,281 13,108 12,206
Conservation programs      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Up to 1 year    
Regulatory Assets, Current $ 7,857 6,963 7,776
Pension and postretirement benefits      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 **    
Regulatory Assets, Noncurrent $ 157,015 157,069 165,843
Asset retirement obligations      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Over plant lives    
Regulatory Assets, Noncurrent $ 68,815 66,000 64,771
Plant to be retired      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 -    
Regulatory Assets, Noncurrent $ 57,499 32,931 26,152
Manufactured gas plant site remediation      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 -    
Regulatory Assets, Noncurrent $ 25,964 15,126 15,510
Taxes recoverable from customers      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Over plant lives    
Regulatory Assets, Noncurrent $ 10,847 11,486 11,600
Long-term debt refinancing costs      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Up to 17 years    
Regulatory Assets, Noncurrent $ 3,826 4,286 4,439
Other Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets, Current 14,613 7,539 13,316
Regulatory Assets, Noncurrent $ 6,840 $ 7,397 $ 8,660
Minimum [Member] | Natural gas costs recoverable through rate adjustments      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Up to 1 year    
Minimum [Member] | Cost recovery mechanisms      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Up to 1 year    
Minimum [Member] | Other Regulatory Assets      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Up to 1 year    
Maximum [Member] | Natural gas costs recoverable through rate adjustments      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Up to 3 years    
Maximum [Member] | Cost recovery mechanisms      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Up to 10 years    
Maximum [Member] | Other Regulatory Assets      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of September 30, 2020 Up to 19 years