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Financial instruments - Additional Information (Detail)
 12 Months Ended
Oct. 25, 2016
USD ($)
Dec. 31, 2017
CAD
MMBTU
CAD / MWh
Dec. 31, 2017
USD ($)
Dec. 31, 2016
CAD
Dec. 31, 2017
USD ($)
MMBTU
CAD / MWh
Jan. 31, 2017
CAD
Jan. 31, 2017
USD ($)
Sep. 29, 2016
CAD
Sep. 29, 2016
USD ($)
Jan. 31, 2014
CAD
Dec. 31, 2012
CAD
Derivative [Line Items]                      
Commodity volumes, Gas | MMBTU   15,457,678     15,457,678            
Effective portion of cash flow hedge, gain (loss)   CAD 8,714,000   CAD 34,355,000              
Other comprehensive income, foreign currency loss   (21,648,000)   0              
Other comprehensive income, foreign currency translation adjustment gain (loss)   (256,067,000)   (67,855,000)              
Issuance of convertible debentures, net of costs   743,881,000   357,694,000              
Total realized loss (gain) on derivative financial instruments   CAD (16,845,000)   3,289,000              
Revenue collection period   45 days 45 days                
Cash on hand   CAD 54,550,000                  
Available for drawn on senior debt facilities   1,145,859,000                  
Utility Services                      
Derivative [Line Items]                      
Accounts receivable balances | $         $ 204,380,000            
Not designated as a hedge                      
Derivative [Line Items]                      
Gain (loss) on derivative   (13,414,000)   20,656,000              
Total realized loss (gain) on derivative financial instruments   (16,950,000)   420,000              
Liberty Power Group                      
Derivative [Line Items]                      
Other comprehensive income, foreign currency translation adjustment gain (loss)   CAD 23,381,000   6,156,000              
Bonds                      
Derivative [Line Items]                      
Par value | $ $ 500,000,000                    
Bonds | Plan                      
Derivative [Line Items]                      
Debt instrument, term   10 years 10 years                
Long-term debt   CAD 135,000,000                  
Senior Unsecured Notes                      
Derivative [Line Items]                      
Long-term debt   3,863,296,000   3,913,415,000              
Senior Unsecured Notes | Convertible Unsecured Subordinated Debentures                      
Derivative [Line Items]                      
Issuance of convertible debentures, net of costs | $     $ 359,950,000                
Senior Unsecured Notes | Liberty Power Group | 4.82% Senior Unsecured Notes                      
Derivative [Line Items]                      
Par value                     CAD 150,000,000
Senior Unsecured Notes | Liberty Power Group | 4.65% Senior Unsecured Notes                      
Derivative [Line Items]                      
Par value                   CAD 200,000,000  
Senior Unsecured Notes | Liberty Power Group | Convertible Unsecured Subordinated Debentures                      
Derivative [Line Items]                      
Par value           CAD 300,000,000          
Foreign exchange forward                      
Derivative [Line Items]                      
Notional amount           CAD 744,050,000 $ 567,665,000 CAD 47,225,000 $ 38,400,000    
Foreign exchange forward | Not designated as a hedge                      
Derivative [Line Items]                      
Total realized loss (gain) on derivative financial instruments   CAD (16,413,000)   1,371,000              
Ten Year Foreign Exchange Forward                      
Derivative [Line Items]                      
Notional amount | $ $ 250,000,000                    
Term of forward-starting interest rate swap 10 years                    
Derivative, forward interest rate 1.8395%                    
Thirty Year Foreign Exchange Forward                      
Derivative [Line Items]                      
Notional amount | $ $ 250,000,000                    
Term of forward-starting interest rate swap 30 years                    
Derivative, forward interest rate 2.5539%                    
Forward contracts                      
Derivative [Line Items]                      
Commodity volumes, Gas | MMBTU   12,420,000     12,420,000            
Other comprehensive income, foreign currency loss   CAD 432,000                  
Interest rate swaps | Not designated as a hedge                      
Derivative [Line Items]                      
Total realized loss (gain) on derivative financial instruments   193,000   0              
Energy contracts | Not designated as a hedge                      
Derivative [Line Items]                      
Total realized loss (gain) on derivative financial instruments   CAD (730,000)   (951,000)              
Cash flow hedge | Foreign exchange forward                      
Derivative [Line Items]                      
Effective portion of cash flow hedge, gain (loss) | $     718,000                
Gain (loss) on derivative | $     $ 36,667,000                
Cash flow hedge | Interest rate swaps                      
Derivative [Line Items]                      
Term of forward-starting interest rate swap   10 years 10 years                
Effective portion of cash flow hedge, gain (loss)   CAD 2,771,000   (3,726,000)              
Minimum                      
Derivative [Line Items]                      
Forward price | CAD / MWh   22.13     22.13            
Maximum                      
Derivative [Line Items]                      
Forward price | CAD / MWh   121.56     121.56            
Weighted Average                      
Derivative [Line Items]                      
Forward price | CAD / MWh   33.20     33.20            
Fair Value                      
Derivative [Line Items]                      
Derivative financial instruments, assets   CAD 87,531,000   150,870,000              
Fair Value | Energy contracts | Not designated as a hedge                      
Derivative [Line Items]                      
Derivative financial instruments, assets   137,000                  
Fair Value | Cash flow hedge | Foreign exchange forward | Not designated as a hedge                      
Derivative [Line Items]                      
Derivative financial instruments, assets       17,864,000              
Carrying amount                      
Derivative [Line Items]                      
Derivative financial instruments, assets   87,531,000   150,870,000              
Carrying amount | Energy contracts | Not designated as a hedge                      
Derivative [Line Items]                      
Derivative financial instruments, assets   CAD 137,000                  
Carrying amount | Cash flow hedge | Foreign exchange forward | Not designated as a hedge                      
Derivative [Line Items]                      
Derivative financial instruments, assets       17,864,000              
Credit Concentration Risk | Accounts Receivable | Liberty Power Group                      
Derivative [Line Items]                      
Major customer revenue percentage   90.00% 90.00%                
Empire | Forward contracts                      
Derivative [Line Items]                      
Other comprehensive income, foreign currency loss   CAD 16,412,000   CAD (17,684,000)              
Non-regulated Energy Sales [Member]                      
Derivative [Line Items]                      
Unrealized gains currently in accumulated other comprehensive loss to be reclassified into net earnings within next twelve months   11,612,000                  
Interest expense                      
Derivative [Line Items]                      
Unrealized gains currently in accumulated other comprehensive loss to be reclassified into net earnings within next twelve months   CAD 2,643,000