XML 71 R55.htm IDEA: XBRL DOCUMENT v3.10.0.1
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