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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Derivative [Line Items] |  |
Regulatory Assets | $ 845,471 | $ 559,887
Regulatory liabilities, natural gas derivative contracts | 601,518 | 607,378
Swap |  |
Derivative [Line Items] |  |
Regulatory Assets | 228 | 28
Regulatory liabilities, natural gas derivative contracts | 271 | 743
Options |  |
Derivative [Line Items] |  |
Regulatory Assets | 50 | 38
Regulatory liabilities, natural gas derivative contracts | 76 | 0
Forward contracts |  |
Derivative [Line Items] |  |
Regulatory Assets | $ 693 | $ 1,830

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