RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Fair Value of Derivatives (Details) - CAD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (2,582)	$ (3,210)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1,696)	(2,718)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(796)	(515)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(90)	30
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	0	(7)
Recurring basis
Fair Value of Derivatives
Current derivative assets	235	498
Long-term derivative assets	139	119
Current derivative liabilities	(894)	(1,234)
Long-term derivative liabilities	(2,062)	(2,593)
Total net derivative asset/(liability)	(2,582)	(3,210)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	37	47
Long-term derivative assets	113	62
Current derivative liabilities	(452)	(615)
Long-term derivative liabilities	(1,394)	(2,212)
Total net derivative asset/(liability)	(1,696)	(2,718)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	2	22
Long-term derivative assets		5
Current derivative liabilities	(258)	(341)
Long-term derivative liabilities	(540)	(201)
Total net derivative asset/(liability)	(796)	(515)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	196	429
Long-term derivative assets	24	52
Current derivative liabilities	(184)	(273)
Long-term derivative liabilities	(126)	(178)
Total net derivative asset/(liability)	(90)	30
Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	2
Current derivative liabilities	0	(5)
Long-term derivative liabilities	(2)	(2)
Total net derivative asset/(liability)	0	(7)
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	6	24
Long-term derivative assets	0	0
Current derivative liabilities	(10)	(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	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	0
Long-term derivative assets		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	6	24
Long-term derivative assets	0	0
Current derivative liabilities	(10)	(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	0
Total net derivative asset/(liability)	0	0
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	66	114
Long-term derivative assets	129	97
Current derivative liabilities	(726)	(989)
Long-term derivative liabilities	(1,945)	(2,438)
Total net derivative asset/(liability)	(2,476)	(3,216)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	37	47
Long-term derivative assets	113	62
Current derivative liabilities	(452)	(615)
Long-term derivative liabilities	(1,394)	(2,212)
Total net derivative asset/(liability)	(1,696)	(2,718)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	2	22
Long-term derivative assets		5
Current derivative liabilities	(258)	(341)
Long-term derivative liabilities	(540)	(201)
Total net derivative asset/(liability)	(796)	(515)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	27	45
Long-term derivative assets	14	30
Current derivative liabilities	(16)	(28)
Long-term derivative liabilities	(9)	(23)
Total net derivative asset/(liability)	16	24
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	2
Current derivative liabilities	0	(5)
Long-term derivative liabilities	(2)	(2)
Total net derivative asset/(liability)	0	(7)
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	163	360
Long-term derivative assets	10	22
Current derivative liabilities	(158)	(238)
Long-term derivative liabilities	(117)	(155)
Total net derivative asset/(liability)	(102)	(11)
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 assets	0	0
Long-term derivative assets		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	163	360
Long-term derivative assets	10	22
Current derivative liabilities	(158)	(238)
Long-term derivative liabilities	(117)	(155)
Total net derivative asset/(liability)	(102)	(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	0
Total net derivative asset/(liability)	$ 0	$ 0