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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, 2019
Dec. 31, 2018
Dec. 31, 2017
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 $ (11,807) $ (10,229) $ (8,013)
Net changes in sales prices and production costs (3,515) 20,386 7,466
Extensions, discoveries and improved recovery 5,883 2,807 481
Changes in estimated future development costs (1,889) (698) (5,548)
Purchases of proved reserves in place 7,418 396 25,782
Sales of proved reserves in place 0 (55) 0
Revisions of previous reserve estimates (3,384) 2,711 4,245
Accretion of discount 8,062 6,119 3,075
Changes in production timing and other 447 (955) (662)
Net change in income taxes 1,984 (7,061) (4,236)
Net change 3,199 13,421 22,590
Balance - beginning of year 60,326 46,905 24,315
Balance - end of year $ 63,525 $ 60,326 $ 46,905