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Regulatory assets and liabilities Regulatory Assets (Details 1) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2020
Dec. 31, 2019
Jun. 30, 2019
Regulatory Assets [Line Items]      
Regulatory Assets, Current $ 60,557 $ 63,613 $ 80,661
Regulatory Assets, Noncurrent 351,025 353,784 327,164
Total regulatory assets 411,582 417,397 407,825
Regulatory assets not earning a rate of return 296,600 276,500 290,700
Natural gas costs recoverable through rate adjustments      
Regulatory Assets [Line Items]      
Regulatory Assets, Current 42,582 42,823 65,448
Regulatory Assets, Noncurrent 27,184 46,381 29,095
Cost recovery mechanisms      
Regulatory Assets [Line Items]      
Regulatory Assets, Current 7,781 6,288 7,484
Regulatory Assets, Noncurrent $ 13,140 13,108 11,004
Conservation programs      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Up to 1 year    
Regulatory Assets, Current $ 7,256 6,963 6,529
Pension and postretirement benefits      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 **    
Regulatory Assets, Noncurrent $ 157,033 157,069 165,861
Asset retirement obligations      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Over plant lives    
Regulatory Assets, Noncurrent $ 68,191 66,000 64,215
Plant to be retired      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 -    
Regulatory Assets, Noncurrent $ 49,185 32,931 16,933
Manufactured gas plant site remediation      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 -    
Regulatory Assets, Noncurrent $ 14,826 15,126 15,387
Taxes recoverable from customers      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Over plant lives    
Regulatory Assets, Noncurrent $ 10,890 11,486 11,755
Long-term debt refinancing costs      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Up to 17 years    
Regulatory Assets, Noncurrent $ 3,980 4,286 4,592
Other Regulatory Assets      
Regulatory Assets [Line Items]      
Regulatory Assets, Current 2,938 7,539 1,200
Regulatory Assets, Noncurrent $ 6,596 $ 7,397 $ 8,322
Minimum [Member] | Natural gas costs recoverable through rate adjustments      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Up to 1 year    
Minimum [Member] | Cost recovery mechanisms      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Up to 1 year    
Minimum [Member] | Other Regulatory Assets      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Up to 1 year    
Maximum [Member] | Natural gas costs recoverable through rate adjustments      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Up to 3 years    
Maximum [Member] | Cost recovery mechanisms      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Up to 10 years    
Maximum [Member] | Other Regulatory Assets      
Regulatory Assets [Line Items]      
Estimated Recovery Period as of June 30, 2020 Up to 19 years