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Decommissioning and Other Provisions - Change in Decommissioning and Other Provision Balances (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Reconciliation of changes in other provisions [abstract]    
Balance $ 470 $ 343
Liabilities incurred 22 22
Liabilities settled (41) (50)
Liabilities disposed   (8)
Accretion (24) 23
Acquisition of liabilities (Big Level) 8  
Revisions in estimated cash flows(2) 5 42
Revisions in discount rates(2) (37) 110
Reversals (5) (4)
Change in foreign exchange rates 10 (8)
Balance 456 470
Decommissioning and restoration    
Reconciliation of changes in other provisions [abstract]    
Balance 437 293
Liabilities incurred 5 3
Liabilities settled (31) (19)
Liabilities disposed   (8)
Accretion (24) 23
Revisions in estimated cash flows(2) 2 41
Revisions in discount rates(2) (37) 110
Reversals 0 0
Change in foreign exchange rates 7 (6)
Balance $ 407 437
Decommissioning and restoration | Minimum    
Reconciliation of changes in other provisions [abstract]    
Use of lower, short-term discount rate (in years) 5 years  
Increase (decrease) in discount rates, other provisions (1.60%)  
Decommissioning and restoration | Maximum    
Reconciliation of changes in other provisions [abstract]    
Use of lower, short-term discount rate (in years) 15 years  
Increase (decrease) in discount rates, other provisions (2.10%)  
Other    
Reconciliation of changes in other provisions [abstract]    
Balance $ 33 50
Liabilities incurred 17 19
Liabilities settled (10) (31)
Liabilities disposed   0
Accretion 0 0
Acquisition of liabilities (Big Level) 8  
Revisions in estimated cash flows(2) 3 1
Revisions in discount rates(2) 0 0
Reversals (5) (4)
Change in foreign exchange rates 3 (2)
Balance $ 49 $ 33