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Significant accounting policies - Additional Information (Detail) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Jan. 01, 2018 USD ($) | Dec. 31, 2019 USD ($) MWh Facility | Dec. 31, 2018 USD ($)
Significant Accounting Policies [Line Items] |  |  |
Number of electric generating facilities | Facility |  | 3 |
Number of power generating facilities | Facility |  | 2 |
Non-regulated energy sales |  | $ 1,624,921 | $ 1,648,463
Interest expense on long-term debt and others |  | $ 181,488 | 152,118
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 |  | $ 60,230 | 59,288
Long-term debt of Long Sault |  | 21,754 | 22,263
Operating expenses and amortization |  | 4,930 | 4,634
Interest expense on long-term debt and others |  | $ 2,340 | 2,557
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 |  |
Adjustment to retained earnings, for previously deferred revenue, net of tax | $ 1,860 |  |
Non-regulated energy sales |  |  |
Significant Accounting Policies [Line Items] |  |  |
Non-regulated energy sales |  | $ 246,601 | 235,359
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 |  | $ 17,108 | $ 17,232

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