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Financial instruments - Impact of Change in Fair Value of Natural Gas Derivative Contracts (Detail) CAD in Thousands, $ in Thousands | Dec. 31, 2017 CAD | Dec. 31, 2017 USD ($) | Dec. 31, 2016 CAD | Dec. 31, 2016 USD ($)
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Derivative [Line Items] |  |  |  |
Regulatory assets, natural gas derivative contracts | CAD | CAD 551,134 |  | CAD 291,964 |
Regulatory liabilities, natural gas derivative contracts | CAD | CAD 725,056 |  | CAD 182,734 |
Swap |  |  |  |
Derivative [Line Items] |  |  |  |
Regulatory assets, natural gas derivative contracts |  | $ 0 |  | $ 0
Regulatory liabilities, natural gas derivative contracts |  | 287 |  | 175
Options |  |  |  |
Derivative [Line Items] |  |  |  |
Regulatory assets, natural gas derivative contracts |  | 0 |  | 27
Regulatory liabilities, natural gas derivative contracts |  | 138 |  | 92
Forward contracts |  |  |  |
Derivative [Line Items] |  |  |  |
Regulatory assets, natural gas derivative contracts |  | 6,319 |  | 0
Regulatory liabilities, natural gas derivative contracts |  | $ 20,909 |  | $ 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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