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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
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Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | $ 753,641 | $ 559,887
Regulatory liabilities, natural gas derivative contracts | 608,826 | 598,062
Swap |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 129 | 28
Regulatory liabilities, natural gas derivative contracts | 139 | 743
Options |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 16 | 38
Regulatory liabilities, natural gas derivative contracts | 25 | 0
Forward contracts |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | $ 809 | $ 1,830

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