RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value of Derivatives (Details) - CAD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (3,098)	$ (1,687)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1,770)	(1,102)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1,237)	(534)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(84)	(58)
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(7)	7
Recurring basis
Fair Value of Derivatives
Current derivative assets	350	328
Long-term derivative assets	308	101
Current derivative liabilities	(1,756)	(936)
Long-term derivative liabilities	(2,000)	(1,180)
Total net derivative asset/(liability)	(3,098)	(1,687)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	73	161
Long-term derivative assets	245	81
Current derivative liabilities	(564)	(410)
Long-term derivative liabilities	(1,524)	(934)
Total net derivative asset/(liability)	(1,770)	(1,102)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative liabilities	(847)	(353)
Long-term derivative liabilities	(390)	(181)
Total net derivative asset/(liability)	(1,237)	(534)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	277	163
Long-term derivative assets	63	17
Current derivative liabilities	(344)	(173)
Long-term derivative liabilities	(80)	(65)
Total net derivative asset/(liability)	(84)	(58)
Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets		4
Long-term derivative assets		3
Current derivative liabilities	(1)
Long-term derivative liabilities	(6)
Total net derivative asset/(liability)	(7)	7
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	15	0
Long-term derivative assets	21	0
Current derivative liabilities	(29)	(5)
Long-term derivative liabilities	(12)	0
Total net derivative asset/(liability)	(5)	(5)
Level 1 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	0	0
Long-term derivative assets	0	0
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	0	0
Level 1 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	0	0
Level 1 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	15	0
Long-term derivative assets	21	0
Current derivative liabilities	(29)	(5)
Long-term derivative liabilities	(12)	0
Total net derivative asset/(liability)	(5)	(5)
Level 1 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets		0
Long-term derivative assets		0
Current derivative liabilities	0
Long-term derivative liabilities	0
Total net derivative asset/(liability)	0	0
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	120	198
Long-term derivative assets	272	96
Current derivative liabilities	(1,428)	(786)
Long-term derivative liabilities	(1,929)	(1,121)
Total net derivative asset/(liability)	(2,965)	(1,613)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	73	161
Long-term derivative assets	245	81
Current derivative liabilities	(564)	(410)
Long-term derivative liabilities	(1,524)	(934)
Total net derivative asset/(liability)	(1,770)	(1,102)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative liabilities	(847)	(353)
Long-term derivative liabilities	(390)	(181)
Total net derivative asset/(liability)	(1,237)	(534)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	47	33
Long-term derivative assets	27	12
Current derivative liabilities	(16)	(23)
Long-term derivative liabilities	(9)	(6)
Total net derivative asset/(liability)	49	16
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets		4
Long-term derivative assets		3
Current derivative liabilities	(1)
Long-term derivative liabilities	(6)
Total net derivative asset/(liability)	(7)	7
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	215	130
Long-term derivative assets	15	5
Current derivative liabilities	(299)	(145)
Long-term derivative liabilities	(59)	(59)
Total net derivative asset/(liability)	(128)	(69)
Level 3 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	0	0
Long-term derivative assets	0	0
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	0	0
Level 3 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative liabilities	0	0
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	0	0
Level 3 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	215	130
Long-term derivative assets	15	5
Current derivative liabilities	(299)	(145)
Long-term derivative liabilities	(59)	(59)
Total net derivative asset/(liability)	(128)	(69)
Level 3 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative assets		0
Long-term derivative assets		0
Current derivative liabilities	0
Long-term derivative liabilities	0
Total net derivative asset/(liability)	$ 0	$ 0