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Regulatory assets and liabilities (Details) - USD ($) $ in Thousands | 6 Months Ended |
--- | --- | ---
Jun. 30, 2019 | Dec. 31, 2018
Regulatory Assets and Liabilities |  |
Regulatory Assets | $ 407,674 | $ 340,117
Regulatory Liabilities | 505,687 | 521,432
Net Regulatory Position | (98,013) | (181,315)
Regulatory assets not earning a rate of return | 290,500 | 313,500
Taxes recoverable from/refundable to customers |  |
Regulatory Assets and Liabilities |  |
Regulatory Liabilities | 264,025 | 277,833
Plant removal |  |
Regulatory Assets and Liabilities |  |
Regulatory Liabilities | 175,003 | 173,143
Natural gas costs refundable through rate adjustments |  |
Regulatory Assets and Liabilities |  |
Regulatory Liabilities | 25,737 | 29,995
Pension and postretirement benefits |  |
Regulatory Assets and Liabilities |  |
Regulatory Liabilities | 12,967 | 15,264
Other Regulatory Assets/Liabilities |  |
Regulatory Assets and Liabilities |  |
Regulatory Liabilities | $ 27,955 | 25,197
Pension and postretirement benefits |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | (e) |
Regulatory Assets | $ 165,861 | 165,898
Natural gas costs recoverable through rate adjustments |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | Up to 3 years |
Regulatory Assets | $ 94,543 | 42,652
Asset Retirement Obligation Costs |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | Over plant lives |
Regulatory Assets | $ 64,215 | 60,097
Cost recovery mechanisms |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | Up to 4 years |
Regulatory Assets | $ 18,488 | 17,948
Manufactured gas plant site remediation |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | - |
Regulatory Assets | $ 15,619 | 17,068
Taxes recoverable from/refundable to customers |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | Over plant lives |
Regulatory Assets | $ 11,755 | 11,946
Plant removal |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | - |
Regulatory Assets | $ 16,933 | 0
Conservation programs |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | Up to 1 year |
Regulatory Assets | $ 7,627 | 7,494
Long-term debt refinancing costs |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | Up to 19 years |
Regulatory Assets | $ 4,592 | 4,898
Costs related to identifying generation development |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | Up to 8 years |
Regulatory Assets | $ 2,280 | 2,508
Other Regulatory Assets/Liabilities |  |
Regulatory Assets and Liabilities |  |
Estimated Recovery Period | Up to 20 years |
Regulatory Assets | $ 5,761 | $ 9,608

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