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OTHER LONG-TERM LIABILITIES - Asset Retirement Obligations (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of other provisions [line items]      
Asset retirement obligations settlement period 60 years    
Reconciliation Of Changes In Provision For Decommissioning Restoration And Rehabilitation Costs [Roll Forward]      
Balance – beginning of year $ 3,243 $ 2,950 $ 4,221
Balance – end of year 4,327 3,243 2,950
Less: current portion 92 95 101
Non-current asset retirement obligation $ 4,235 $ 3,148 $ 2,849
Provision for decommissioning, restoration and rehabilitation costs [member]      
Disclosure of other provisions [line items]      
Weighted average discount rate 4.70% 5.20% 5.90%
Reconciliation Of Changes In Provision For Decommissioning Restoration And Rehabilitation Costs [Roll Forward]      
Liabilities incurred $ 12 $ 3 $ 7
Liabilities acquired, net 784 30 129
Liabilities settled (274) (267) (370)
Asset retirement obligation accretion 164 142 173
Revision of cost, inflation rates and timing estimates (40) (68) (313)
Change in discount rate 509 493 (1,150)
Foreign exchange adjustments $ (71) $ (40) $ 253