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Significant accounting policies - Additional Information (Detail) | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) Facility | Dec. 31, 2017 USD ($) MWh
Significant Accounting Policies [Line Items] |  |  |
Number of electric generating facilities | Facility |  | 3 |
Number of power generating facilities | Facility |  | 2 |
Non-regulated energy sales |  | $ 1,647,387,000 | $ 1,521,938,000
Interest expense on long-term debt and others |  | 152,118,000 | $ 155,822,000
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 |  | 59,288,000 | $ 67,398,000
Long-term debt of Long Sault |  | 22,263,000 | 28,628,000
Long-term debt of Long Sault, with recourse |  | 0 | 3,109,000
Operating expenses and amortization |  | 4,634,000 | 4,289,000
Interest expense on long-term debt and others |  | $ 1,258,000 | 2,755,000
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 |
Accounting Standards Update 2014-09 |  |  |
Significant Accounting Policies [Line Items] |  |  |
Adjustment to retained earnings, for previously deferred revenue, before tax | $ 2,488,000 |  |
Adjustment to retained earnings, for previously deferred revenue, net of tax | $ 1,860,000 |  |
Non-regulated energy sales |  |  |
Significant Accounting Policies [Line Items] |  |  |
Non-regulated energy sales |  | $ 235,359,000 | 217,542,000
Non-regulated energy sales | Long Sault and Saint-Damase Wind Powered Generating Facility | Primary Beneficiary | Power plant |  |  |
Significant Accounting Policies [Line Items] |  |  |
Non-regulated energy sales |  | $ 17,232,000 | $ 17,508,000

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