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Regulatory assets and liabilities (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018
Regulatory assets and liabilities |  |
Regulatory assets: | $ 417,397 | $ 340,117
Regulatory liabilities: | 490,305 | 521,432
Net regulatory position | (72,908) | (181,315)
Regulatory assets not earning a rate of return | 276,500 | 313,500
Taxes recoverable from/refundable to customers |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | 249,506 | 277,833
Plants to be retired |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | 173,722 | 173,143
Natural gas costs refundable through rate adjustments |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | 23,825 | 29,995
Pension and postretirement benefits |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | 18,065 | 15,264
Other regulatory assets/liabilities |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | $ 25,187 | 25,197
Pension and postretirement benefits |  |
Regulatory assets and liabilities |  |
Estimated recovery period | (e) |
Regulatory assets: | $ 157,069 | 165,898
Natural gas costs recoverable through rate adjustments |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 3 years |
Regulatory assets: | $ 89,204 | 42,652
Asset retirement obligation costs |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Over plant lives |
Regulatory assets: | $ 66,000 | 60,097
Plants to be retired |  |
Regulatory assets and liabilities |  |
Estimated recovery period | - |
Regulatory assets: | $ 32,931 | 0
Cost recovery mechanisms (a) (b) |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 3 years |
Regulatory assets: | $ 19,396 | 17,948
Manufactured gas plant sites remediation |  |
Regulatory assets and liabilities |  |
Estimated recovery period | - |
Regulatory assets: | $ 15,347 | 17,068
Taxes recoverable from/refundable to customers |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Over plant lives |
Regulatory assets: | $ 11,486 | 11,946
Conservation programs (a) (b) |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 3 years |
Regulatory assets: | $ 7,405 | 7,494
Long-term debt refinancing costs |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 18 years |
Regulatory assets: | $ 4,286 | 4,898
Costs related to identifying generation development |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 7 years |
Regulatory assets: | $ 2,052 | 2,508
Other regulatory assets/liabilities |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 19 years |
Regulatory assets: | $ 12,221 | $ 9,608

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