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SUPPLEMENTARY OIL & GAS INFORMATION (Unaudited) - Sources of Change in the Standardized Measure of Discounted Future Net Cash Flows (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves 1 [Roll Forward]      
Sales of crude oil and natural gas produced, net of production costs $ (10,229) $ (8,013) $ (4,159)
Net changes in sales prices and production costs 20,386 7,466 (7,305)
Extensions, discoveries and improved recovery 2,807 481 700
Changes in estimated future development costs (698) (5,548) 1,750
Purchases of proved reserves in place 396 25,782 352
Sales of proved reserves in place (55) 0 (2)
Revisions of previous reserve estimates 2,711 4,245 3,668
Accretion of discount 6,119 3,075 3,527
Changes in production timing and other (955) (662) (2,137)
Net change in income taxes (7,061) (4,236) 385
Net change 13,421 22,590 (3,221)
Balance - beginning of year 46,905 24,315 27,536
Balance - end of year $ 60,326 $ 46,905 $ 24,315