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INVESTMENTS - PrairieSky (Details) - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of detailed information about financial instruments [line items] |  |  |
(Gain) loss from investment | $ (38) | $ (327) | $ 50
Investment in PrairieSky Royalty Ltd. |  |  |
Disclosure of detailed information about financial instruments [line items] |  |  |
(Gain) loss from investments | (3) | (292) | 11
Dividend income | (17) | (27) | (5)
(Gain) loss from investment | $ (20) | $ (319) | $ 6

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