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Risk Management Activities - Hedged items on the statement of financial position (Details)
MWh in Millions, $ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2021
CAD ($)
Dec. 31, 2020
CAD ($)
Dec. 31, 2021
USD ($)
MWh
Dec. 31, 2021
CAD ($)
MWh
Dec. 31, 2020
USD ($)
MWh
Dec. 31, 2020
CAD ($)
MWh
Disclosure of detailed information about hedging instruments [line items]            
Hedging instrument, assets       $ 2   $ 2
Physical power sales | Cash flow hedges | Commodity risk management            
Disclosure of detailed information about hedging instruments [line items]            
Notional amount purchased | MWh     13 13 16 16
Hedging instrument, assets       $ 285   $ 573
Gain (loss) on hedge ineffectiveness $ (181) $ (33)        
Reserve of cash flow hedges       226   417
Interest Expense On Long-Term Debt | Cash flow hedges | Interest rate risk            
Disclosure of detailed information about hedging instruments [line items]            
Gain (loss) on hedge ineffectiveness 3 7        
Reserve of cash flow hedges       2   19
Foreign-denominated debt | Net Investment Hedge | Currency risk            
Disclosure of detailed information about hedging instruments [line items]            
Hedging instrument, assets       473   472
Gain (loss) on hedge ineffectiveness 0 11        
Foreign-denominated debt | Net Investment Hedge | Currency risk | U.S. Dollars            
Disclosure of detailed information about hedging instruments [line items]            
Notional amount     $ 370   $ 370  
Interest rate swap | Cash flow hedges | Interest rate risk | U.S. Dollars            
Disclosure of detailed information about hedging instruments [line items]            
Notional amount     300   $ 150  
Hedging instrument, liabilities       3   (3)
Gain (loss) on hedge ineffectiveness 3 3        
Interest rate swap | Cash flow hedges | Interest rate risk | Canada, Dollars            
Disclosure of detailed information about hedging instruments [line items]            
Notional amount           75
Hedging instrument, liabilities           $ (4)
Gain (loss) on hedge ineffectiveness   $ 4        
Foreign-Denominated Expenditures | Cash flow hedges | Interest rate risk | U.S. Dollars | Minimum            
Disclosure of detailed information about hedging instruments [line items]            
Notional amount     8      
Hedging instrument, liabilities       0    
Gain (loss) on hedge ineffectiveness 0          
Foreign-Denominated Expenditures | Cash flow hedges | Interest rate risk | U.S. Dollars | Maximum            
Disclosure of detailed information about hedging instruments [line items]            
Notional amount     $ 14      
Hedging instrument, liabilities       $ 0    
Gain (loss) on hedge ineffectiveness $ 0