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Significant accounting policies - Additional Information (Detail) CAD in Thousands, $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 CAD MWh Facility | Dec. 31, 2016 CAD | Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items] |  |  |
Commonly Owned, Number of Electric Generating Facilities | Facility | 3 |  |
Restricted cash held for common shareholders | $ |  |  | $ 1,495,774
Number of power generating facilities | Facility | 2 |  |
Non-regulated energy sales | CAD 282,558 | CAD 243,149 |
Interest expense on long-term debt and others | CAD 202,631 | 131,592 |
Electricity generated from an eligible energy source (megawatt) | MWh | 1 |  |
Long Sault and Saint-Damase Wind Powered Generating Facility | Primary Beneficiary | Power plant |  |  |
Significant Accounting Policies [Line Items] |  |  |
Generating assets of Long Sault | CAD 84,550 | 87,189 |
Long-term debt of Long Sault | 35,914 | 40,398 |
Long-term debt of Long Sault, with recourse | 3,900 | 6,900 |
Non-regulated energy sales | 22,743 | 29,132 |
Operating expenses and amortization | 5,564 | 6,175 |
Interest expense on long-term debt and others | CAD 3,573 | CAD 4,064 |
Minimum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Ownership interest in commonly owned facilities | 7.52% |  |
Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Ownership interest in commonly owned facilities | 60.00% |  |
Power sales contracts | Minimum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Intangible asset, useful life | 6 years |  |
Power sales contracts | Maximum |  |  |
Significant Accounting Policies [Line Items] |  |  |
Intangible asset, useful life | 25 years |  |
Interconnection agreements |  |  |
Significant Accounting Policies [Line Items] |  |  |
Intangible asset, useful life | 40 years |  |
Customer Relationships |  |  |
Significant Accounting Policies [Line Items] |  |  |
Intangible asset, useful life | 40 years |  |

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- Definition Commonly Owned, Number of Electric Generating Facilities + References No definition available. + Details
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- Definition Electricity Generated from an Eligible Energy Source + References No definition available. + Details
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- Definition Ownership Interest, Commonly Owned Facilities + References No definition available. + Details
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- Definition Restricted Cash and Cash Equivalents held for Common Shareholders + References No definition available. + Details
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- Definition Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275 + Details
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- Definition Revenue earned during the period from alternative energy sources (for example, ethanol, biomass, solar, geothermal, and wind power). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 24 -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62246-109447 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 605 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=68059722&loc=SL51755165-203213 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.1) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 932 -SubTopic 235 -Section 50 -Paragraph 23 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62136-109447 + Details
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