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Risk Management Activities - Hedged items on the statement of financial position (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of detailed information about hedging instruments [line items]      
Accumulated other comprehensive income $ 302 $ 454  
Reserve of exchange differences on translation      
Disclosure of detailed information about hedging instruments [line items]      
Accumulated other comprehensive income (21) (21) $ 17
Physical power sales | Cash flow hedges | Commodity risk management      
Disclosure of detailed information about hedging instruments [line items]      
Gain (loss) on hedge ineffectiveness (33) 47  
Reserve of cash flow hedges 417 527  
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 7 0  
Reserve of cash flow hedges 19 0  
Foreign-denominated debt | Net Investment Hedge | Currency risk      
Disclosure of detailed information about hedging instruments [line items]      
Gain (loss) on hedge ineffectiveness $ 11 $ 21