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Risk Management Activities - Hedging Instruments (Details)
$ in Thousands, $ in Thousands, $ in Millions, kJ in Billions
 12 Months Ended
Dec. 31, 2018
CAD ($)
MWh
kJ
Dec. 31, 2017
CAD ($)
MWh
kJ
Dec. 31, 2016
CAD ($)
Dec. 31, 2018
USD ($)
MWh
kJ
Dec. 31, 2018
AUD ($)
MWh
kJ
Dec. 31, 2017
USD ($)
MWh
kJ
Disclosure of detailed information about financial instruments [line items]            
Collateral paid (Note 15) $ 105,000 $ 67,000        
Notional amount 3,234,000 3,664,000        
Fair value asset 10,000 30,000        
Unrealized pre-tax gains (losses) 4,000 $ 2,000 $ 0      
Borrowings subject to floating interest rates   14.44032% 6.00%     14.44032%
Gains (losses) on exchange differences on translation, net of tax $ 43,000 $ (25,000)        
Electricity | Non-Hedges | Commodity risk management            
Disclosure of detailed information about financial instruments [line items]            
Notional amount sold | MWh 58,885,000 14,688,000   58,885,000 58,885,000 14,688,000
Notional amount purchased | MWh 37,023,000 7,348,000   37,023,000 37,023,000 7,348,000
Electricity | Cash flow hedges | Commodity risk management            
Disclosure of detailed information about financial instruments [line items]            
Notional amount sold | MWh 2,128,000 1,997,000   2,128,000 2,128,000 1,997,000
Notional amount purchased | MWh 0 44,000   0 0 44,000
Long-term debt | Net investment hedges            
Disclosure of detailed information about financial instruments [line items]            
Notional amount       $ 400,000   $ 480,000
Long-term debt | Fair value hedges | Interest rate risk            
Disclosure of detailed information about financial instruments [line items]            
Notional amount       50,000   $ 50,000
Physical power sales | Cash flow hedges | Commodity risk management            
Disclosure of detailed information about financial instruments [line items]            
Notional amount       $ 400    
Fair value asset $ 687,000          
Notional amount purchased | MWh 23,000     23,000 23,000  
Gain (loss) on hedge ineffectiveness $ 60,000          
Reserve of cash flow hedges 508,000          
Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk            
Disclosure of detailed information about financial instruments [line items]            
Fair value asset   $ 11,000        
Fair value liability 5,000 9,000        
Foreign Exchange Forward Contracts - foreign-denominated debt | Discontinued Hedge Positions | Currency risk            
Disclosure of detailed information about financial instruments [line items]            
Notional amount sold 124,000          
Notional amount purchased   230   $ 100,000    
Fair value asset 10,000          
Fair value liability   4,000        
Cross-Currency Swaps - foreign-denominated debt | Discontinued Hedge Positions | Currency risk            
Disclosure of detailed information about financial instruments [line items]            
Notional amount sold 0          
Notional amount purchased   270   $ 0    
Fair value asset $ 0 $ 35,000        
Natural gas | Non-Hedges | Commodity risk management            
Disclosure of detailed information about financial instruments [line items]            
Notional amount sold | kJ 80,413 74,194   80,413 80,413 74,194
Notional amount purchased | kJ 110,488 103,805   110,488 110,488 103,805
Transmission | Non-Hedges | Commodity risk management            
Disclosure of detailed information about financial instruments [line items]            
Notional amount sold | MWh 29,000 1,000   29,000 29,000 1,000
Notional amount purchased | MWh 11,163,000 3,455,000   11,163,000 11,163,000 3,455,000
Foreign-denominated debt | Net Investment Hedge | Currency risk            
Disclosure of detailed information about financial instruments [line items]            
Fair value asset $ 1,000          
Gain (loss) on hedge ineffectiveness 41,000          
Minimum | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk            
Disclosure of detailed information about financial instruments [line items]            
Notional amount purchased 214,000 $ 157,000        
Maximum | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk            
Disclosure of detailed information about financial instruments [line items]            
Notional amount purchased 205,000 104,000        
Fair value liability 7,000          
U.S. Dollars | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk            
Disclosure of detailed information about financial instruments [line items]            
Notional amount sold       $ 164,000    
Australia, Dollars | Foreign Exchange Forward Contracts - foreign-denominated receipts/expenditures | Discontinued Hedge Positions | Currency risk            
Disclosure of detailed information about financial instruments [line items]            
Notional amount sold         $ 218  
Reserve of exchange differences on translation            
Disclosure of detailed information about financial instruments [line items]            
Gains (losses) on exchange differences on translation, net of tax $ (84,000) $ 89,000