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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Derivative [Line Items] |  |
Regulatory Assets | $ 1,115,098 | $ 845,471
Regulatory liabilities, natural gas derivative contracts | 580,630 | 601,518
Swap |  |
Derivative [Line Items] |  |
Regulatory Assets | 911 | 228
Regulatory liabilities, natural gas derivative contracts | 0 | 271
Options |  |
Derivative [Line Items] |  |
Regulatory Assets | 68 | 50
Regulatory liabilities, natural gas derivative contracts | 0 | 76
Forward contracts |  |
Derivative [Line Items] |  |
Regulatory Assets | 0 | 693
Regulatory liabilities, natural gas derivative contracts | $ 197 | $ 0

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