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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
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Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | $ 446,723 | $ 450,474
Regulatory liabilities, natural gas derivative contracts | 581,897 | 578,592
Swap |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 119 | 66
Regulatory liabilities, natural gas derivative contracts | 645 | 218
Option contracts |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 30 | 0
Regulatory liabilities, natural gas derivative contracts | 0 | 134
Forward contracts |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 989 | 0
Regulatory liabilities, natural gas derivative contracts | $ 0 | $ 1,259

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