RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value of Derivatives (Details) - CAD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (2,860)	$ (3,210)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(2,110)	(2,718)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(517)	(515)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(234)	30
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	1	(7)
Recurring basis
Fair Value of Derivatives
Current derivative assets	205	498
Long-term derivative assets	87	119
Current derivative liabilities	(946)	(1,234)
Long-term derivative liabilities	(2,206)	(2,593)
Total net derivative asset/(liability)	(2,860)	(3,210)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	26	47
Long-term derivative assets	50	62
Current derivative liabilities	(506)	(615)
Long-term derivative liabilities	(1,680)	(2,212)
Total net derivative asset/(liability)	(2,110)	(2,718)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	7	22
Long-term derivative assets		5
Current derivative liabilities	(150)	(341)
Long-term derivative liabilities	(374)	(201)
Total net derivative asset/(liability)	(517)	(515)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	172	429
Long-term derivative assets	35	52
Current derivative liabilities	(289)	(273)
Long-term derivative liabilities	(152)	(178)
Total net derivative asset/(liability)	(234)	30
Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	2
Current derivative liabilities	(1)	(5)
Long-term derivative liabilities		(2)
Total net derivative asset/(liability)	1	(7)
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	3	24
Long-term derivative assets	0	0
Current derivative liabilities	(7)	(7)
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	(4)	17
Level 1 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets		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 assets		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 | Commodity contracts
Fair Value of Derivatives
Current derivative assets	3	24
Long-term derivative assets	0	0
Current derivative liabilities	(7)	(7)
Long-term derivative liabilities	0	0
Total net derivative asset/(liability)	(4)	17
Level 1 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	0
Current derivative liabilities	0	0
Long-term derivative liabilities		0
Total net derivative asset/(liability)	0	0
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	43	114
Long-term derivative assets	65	97
Current derivative liabilities	(673)	(989)
Long-term derivative liabilities	(2,069)	(2,438)
Total net derivative asset/(liability)	(2,634)	(3,216)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	26	47
Long-term derivative assets	50	62
Current derivative liabilities	(506)	(615)
Long-term derivative liabilities	(1,680)	(2,212)
Total net derivative asset/(liability)	(2,110)	(2,718)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	7	22
Long-term derivative assets		5
Current derivative liabilities	(150)	(341)
Long-term derivative liabilities	(374)	(201)
Total net derivative asset/(liability)	(517)	(515)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	10	45
Long-term derivative assets	13	30
Current derivative liabilities	(16)	(28)
Long-term derivative liabilities	(15)	(23)
Total net derivative asset/(liability)	(8)	24
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	2
Current derivative liabilities	(1)	(5)
Long-term derivative liabilities		(2)
Total net derivative asset/(liability)	1	(7)
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	159	360
Long-term derivative assets	22	22
Current derivative liabilities	(266)	(238)
Long-term derivative liabilities	(137)	(155)
Total net derivative asset/(liability)	(222)	(11)
Level 3 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets		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 assets		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 | Commodity contracts
Fair Value of Derivatives
Current derivative assets	159	360
Long-term derivative assets	22	22
Current derivative liabilities	(266)	(238)
Long-term derivative liabilities	(137)	(155)
Total net derivative asset/(liability)	(222)	(11)
Level 3 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	0
Current derivative liabilities	0	0
Long-term derivative liabilities		0
Total net derivative asset/(liability)	$ 0	$ 0