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Other long-term liabilities and deferred credits - Schedule of Long-Term Liabilities and Deferred Credits (Detail) - CAD
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other Long-term Liabilities      
Advances in aid of construction (a) CAD 78,636,000 CAD 105,191,000  
Environmental remediation obligation (b) 68,147,000 63,378,000 CAD 71,529,000
Asset retirement obligations (c) 55,406,000 24,822,000  
Customer deposits (d) 35,790,000 14,881,000  
Unamortized investment tax credits (e) 22,379,000 0  
Deferred credits (f) 26,555,000 44,544,000  
Other 55,779,000 22,790,000  
Other Liabilities 342,692,000 275,606,000  
Less current portion (57,586,000) (43,157,000)  
Other long-term liabilities 285,106,000 232,449,000  
Transfers from advances in aid of construction to contributions in aid of construction 13,626,000 23,986,000  
Estimated environmental remediation costs 71,873,000 76,853,000  
Accrual for environmental loss contingencies to be incurred over next three years CAD 25,186,000    
Accrual for environmental loss contingencies, payment period 28 years    
Amount of regulatory assets CAD 551,134,000 291,964,000  
Obligation incurred 2,604,000 393,000  
Changes of estimate 1,476,000 1,022,000  
Accretion expense 2,551,000 1,055,000  
Settlements CAD 5,418,000    
Minimum      
Other Long-term Liabilities      
Other liability repayment period 5 years    
Accrual environmental cost 2.20%    
Maximum      
Other Long-term Liabilities      
Other liability repayment period 40 years    
Accrual environmental cost 2.50%    
Environmental costs      
Other Long-term Liabilities      
Regulatory asset, expenditure recovery provided by regulator, time period 7 years    
Amount of regulatory assets CAD 103,761,000 CAD 104,160,000  
Empire      
Other Long-term Liabilities      
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Asset Retirement Obligation 31,717,000    
Deerfield SponsorCo      
Other Long-term Liabilities      
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Asset Retirement Obligation CAD 2,816,000