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Significant accounting policies - Additional Information (Detail) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($) waterUtility facility | Dec. 31, 2019 USD ($)
Significant Accounting Policies [Line Items] |  |
Number of electric generating facilities | facility | 3 |
Number of power generating facilities | facility | 2 |
Number of water pipeline projects | waterUtility | 1 |
Generating assets of Long Sault | $ 13,223,906 | $ 10,920,786
Long-term debt of Long Sault | 7,234,541 | 6,182,415
Non-regulated energy sales | 1,677,058 | 1,626,392
Interest expense on long-term debt and others | 181,934 | 181,488
Long Sault and Saint-Damase Wind Powered Generating Facility | Primary Beneficiary | Power plant |  |
Significant Accounting Policies [Line Items] |  |
Generating assets of Long Sault | 59,521 | 60,230
Long-term debt of Long Sault | 20,328 | 21,754
Operating expenses and amortization | 5,400 | 4,930
Interest expense on long-term debt and others | $ 2,119 | 2,340
Minimum |  |
Significant Accounting Policies [Line Items] |  |
Ownership interest in commonly owned facilities | 7.52% |
Lease renewal term | 1 year |
Maximum |  |
Significant Accounting Policies [Line Items] |  |
Ownership interest in commonly owned facilities | 60.00% |
Lease renewal term | 5 years |
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 | Minimum |  |
Significant Accounting Policies [Line Items] |  |
Intangible asset, useful life | 25 years |
Customer Relationships | Maximum |  |
Significant Accounting Policies [Line Items] |  |
Intangible asset, useful life | 40 years |
Non-regulated energy sales |  |
Significant Accounting Policies [Line Items] |  |
Non-regulated energy sales | $ 255,955 | 246,601
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 | $ 17,116 | $ 17,108

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