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Risk Management Activities - Net Risk Management Assets and Liabilities (Details) - CAD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Financial Assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic $ 677 $ 762
Not designated as a hedge | Financial Liabilities    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (10)  
Not designated as a hedge | Financial Assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   52
Commodity risk management    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   728
Commodity risk management | Financial Assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 679  
Commodity risk management | Current    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 59 81
Commodity risk management | Long-term    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 620 647
Commodity risk management | Not designated as a hedge    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   18
Commodity risk management | Not designated as a hedge | Financial Liabilities    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (8)  
Commodity risk management | Not designated as a hedge | Current    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0  
Commodity risk management | Not designated as a hedge | Long-term    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (8) 11
Commodity risk management | Not designated as a hedge | Current    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   7
Other    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   34
Other | Financial Assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (2)  
Other | Current    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (3) 37
Other | Long-term    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 1 (3)
Other | Not designated as a hedge | Financial Liabilities    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   34
Other | Not designated as a hedge | Current    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   37
Other | Not designated as a hedge | Long-term    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   (3)
Other | Not designated as a hedge | Financial Assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (2)  
Other | Not designated as a hedge | Current    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic (3)  
Other | Not designated as a hedge | Long-term    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 1  
Cash flow hedges | Designated as a hedge | Financial Assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 687 710
Cash flow hedges | Commodity risk management | Designated as a hedge    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   710
Cash flow hedges | Commodity risk management | Designated as a hedge | Financial Assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 687  
Cash flow hedges | Commodity risk management | Designated as a hedge | Current    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 59 74
Cash flow hedges | Commodity risk management | Designated as a hedge | Long-term    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 628 636
Cash flow hedges | Other | Designated as a hedge    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic   0
Cash flow hedges | Other | Designated as a hedge | Financial Assets    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0  
Cash flow hedges | Other | Designated as a hedge | Current    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic 0 0
Cash flow hedges | Other | Designated as a hedge | Long-term    
Disclosure of detailed information about financial instruments [line items]    
Risk exposure associated with instruments sharing characteristic $ 0 $ 0