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Decommissioning and Other Provisions - Change in Decommissioning and Other Provision Balances (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Reconciliation of changes in other provisions [abstract]    
Balance $ 546 $ 456
Liabilities incurred 35 14
Liabilities settled (37) (43)
Accretion (30) 23
Acquisition of liabilities 1 19
Disposition of liabilities   (32)
Revisions in estimated cash flows 72 103
Revisions in discount rates 36 16
Reversals (6) (1)
Change in foreign exchange rates (4) (7)
Balance $ 673 546
Minimum    
Reconciliation of changes in other provisions [abstract]    
Increase (decrease) in discount rates, other provisions (0.30%)  
Maximum    
Reconciliation of changes in other provisions [abstract]    
Increase (decrease) in discount rates, other provisions (0.90%)  
IFRS 16    
Reconciliation of changes in other provisions [abstract]    
Balance   2
Decommissioning and restoration    
Reconciliation of changes in other provisions [abstract]    
Balance $ 501 407
Liabilities incurred 1 7
Liabilities settled (18) (34)
Accretion (30) 23
Acquisition of liabilities 1 16
Disposition of liabilities   (23)
Revisions in estimated cash flows 61 96
Revisions in discount rates 36 16
Reversals 0 0
Change in foreign exchange rates (4) (7)
Balance 608 501
Decommissioning and restoration | IFRS 16    
Reconciliation of changes in other provisions [abstract]    
Balance   0
Other    
Reconciliation of changes in other provisions [abstract]    
Balance 45 49
Liabilities incurred 34 7
Liabilities settled (19) (9)
Accretion 0 0
Acquisition of liabilities 0 3
Disposition of liabilities   (9)
Revisions in estimated cash flows 11 7
Revisions in discount rates 0 0
Reversals (6) (1)
Change in foreign exchange rates 0 0
Balance $ 65 45
Other | IFRS 16    
Reconciliation of changes in other provisions [abstract]    
Balance   $ 2