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Risk Management Activities - Net Risk Management Assets and Liabilities (Details) - CAD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   $ 692
Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   4
Current | Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic $ 90 85
Current | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (13) 0
Long-term | Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 452 607
Long-term | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 1 4
Financial Assets    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 530 696
Financial Assets | Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 542  
Financial Assets | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (12)  
Designated as a hedge | Cash flow hedges | Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   676
Designated as a hedge | Cash flow hedges | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   0
Designated as a hedge | Current | Cash flow hedges | Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 101 70
Designated as a hedge | Current | Cash flow hedges | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (9) 0
Designated as a hedge | Long-term | Cash flow hedges | Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 471 606
Designated as a hedge | Long-term | Cash flow hedges | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Designated as a hedge | Financial Assets | Cash flow hedges    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 563 676
Designated as a hedge | Financial Assets | Cash flow hedges | Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 572  
Designated as a hedge | Financial Assets | Cash flow hedges | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (9)  
Not designated as a hedge | Financial Liabilities    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (33)  
Not designated as a hedge | Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   16
Not designated as a hedge | Commodity risk management | Long-term    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (19) 1
Not designated as a hedge | Commodity risk management | Current    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (11)  
Not designated as a hedge | Commodity risk management | Financial Liabilities    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (30)  
Not designated as a hedge | Other | Long-term    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   4
Not designated as a hedge | Other | Current    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   0
Not designated as a hedge | Other | Financial Liabilities    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   4
Not designated as a hedge | Current | Commodity risk management    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   15
Not designated as a hedge | Current | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (4)  
Not designated as a hedge | Long-term | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 1  
Not designated as a hedge | Financial Assets    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   $ 20
Not designated as a hedge | Financial Assets | Other    
Disclosure of nature and extent of risks arising from financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic $ (3)