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Regulatory assets and liabilities (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
Regulatory assets and liabilities |  |
Regulatory assets: | $ 340,117 | $ 300,963
Regulatory liabilities: | 521,432 | 519,298
Net regulatory position | (181,315) | (218,335)
Regulatory assets not earning a rate of return | 313,500 | 269,100
Taxes recoverable from/refundable to customers |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | 277,833 | 279,668
Plant removal and decommissioning costs |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | 173,143 | 176,190
Natural gas costs refundable through rate adjustments |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | 29,995 | 28,514
Pension and postretirement benefits |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | 15,264 | 16,021
Other regulatory assets/liabilities |  |
Regulatory assets and liabilities |  |
Regulatory liabilities: | $ 25,197 | 18,905
Pension and postretirement benefits |  |
Regulatory assets and liabilities |  |
Estimated recovery period | (e) |
Regulatory assets: | $ 165,898 | 163,896
Asset retirement obligation costs |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Over plant lives |
Regulatory assets: | $ 60,097 | 56,078
Natural gas costs recoverable through rate adjustments |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 1 year |
Regulatory assets: | $ 42,652 | 14,465
Taxes recoverable from/refundable to customers |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Over plant lives |
Regulatory assets: | $ 11,946 | 12,073
Manufactured gas plant sites remediation |  |
Regulatory assets and liabilities |  |
Estimated recovery period | - |
Regulatory assets: | $ 16,504 | 18,213
Long-term debt refinancing costs |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 19 years |
Regulatory assets: | $ 4,898 | 5,563
Costs related to identifying generation development |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 8 years |
Regulatory assets: | $ 2,508 | 2,960
Other regulatory assets/liabilities |  |
Regulatory assets and liabilities |  |
Estimated recovery period | Up to 20 years |
Regulatory assets: | $ 35,614 | 27,715
Other noncurrent assets | Stranded costs |  |
Regulatory assets and liabilities |  |
Deferred tax liabilities, regulatory assets |  | 15,500
Other noncurrent liabilities | Stranded costs |  |
Regulatory assets and liabilities |  |
Deferred tax liabilities, regulatory assets |  | $ 270,000

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