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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | $ 559,887 | $ 460,095
Regulatory liabilities, natural gas derivative contracts | 598,062 | 588,213
Swap |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 28 | 66
Regulatory liabilities, natural gas derivative contracts | 743 | 218
Options |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 38 | 0
Regulatory liabilities, natural gas derivative contracts | 0 | 134
Forward contracts |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 1,830 | 0
Regulatory liabilities, natural gas derivative contracts | $ 0 | $ 1,259

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