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INVESTMENTS - Narrative (Details) - CAD ($)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Investment in PrairieSky Royalty Ltd.      
Disclosure of detailed information about financial instruments [line items]      
Non-cash share consideration received   $ 985  
Non-cash share consideration received (in shares)   44.4  
Non-cash share consideration received, price per share (in CAD per share)   $ 22.16  
Ownership interest in investment (less than)     10.00%
Number of shares held as investment (in shares)     22.6
Investment in Inter Pipeline Ltd.      
Disclosure of detailed information about financial instruments [line items]      
Number of shares held as investment (in shares)     6.4
Partial disposal of PrairieSky Royalty Ltd.      
Disclosure of detailed information about financial instruments [line items]      
Non-cash shares distributed (in shares) 21.8    
Return of capital on PrairieSky Royalty Ltd. share distribution $ 546    
Disposal of Cold Lake Pipeline | Investment in Inter Pipeline Ltd.      
Disclosure of detailed information about financial instruments [line items]      
Non-cash share consideration received $ 190    
Non-cash share consideration received (in shares) 6.4    
Non-cash share consideration received, price per share (in CAD per share) $ 29.57