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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | $ 450,474 | $ 441,526
Regulatory liabilities, natural gas derivative contracts | 578,592 | 576,124
Swap |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 66 | 0
Regulatory liabilities, natural gas derivative contracts | 218 | 287
Options |  |
Derivative [Line Items] |  |
Regulatory liabilities, natural gas derivative contracts | 134 | 138
Forward contracts |  |
Derivative [Line Items] |  |
Regulatory assets, natural gas derivative contracts | 0 | 6,319
Regulatory liabilities, natural gas derivative contracts | $ 1,259 | $ 0

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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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