RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - FAIR VALUE OF DERIVATIVES (Details) - CAD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (2,359)	$ (2,192)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1,771)	(1,383)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(286)	(348)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(284)	(457)
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(18)	(4)
Recurring basis
Fair Value of Derivatives
Current derivative assets	261	296
Long-term derivative assets	147	181
Current derivative liabilities	(917)	(1,130)
Long-term derivative liabilities	(1,850)	(1,539)
Total net derivative asset/(liability)	(2,359)	(2,192)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	134	143
Long-term derivative assets	110	145
Current derivative liabilities	(355)	(359)
Long-term derivative liabilities	(1,660)	(1,312)
Total net derivative asset/(liability)	(1,771)	(1,383)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	27	8
Long-term derivative assets	15	13
Current derivative liabilities	(306)	(329)
Long-term derivative liabilities	(22)	(40)
Total net derivative asset/(liability)	(286)	(348)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	100	145
Long-term derivative assets	22	23
Current derivative liabilities	(246)	(439)
Long-term derivative liabilities	(160)	(186)
Total net derivative asset/(liability)	(284)	(457)
Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative liabilities	(10)	(3)
Long-term derivative liabilities	(8)	(1)
Total net derivative asset/(liability)	(18)	(4)
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	0	1
Current derivative liabilities	(6)	(13)
Total net derivative asset/(liability)	(6)	(12)
Level 1 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	0	1
Current derivative liabilities	(6)	(13)
Total net derivative asset/(liability)	(6)	(12)
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	179	181
Long-term derivative assets	126	160
Current derivative liabilities	(759)	(778)
Long-term derivative liabilities	(1,694)	(1,356)
Total net derivative asset/(liability)	(2,148)	(1,793)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	134	143
Long-term derivative assets	110	145
Current derivative liabilities	(355)	(359)
Long-term derivative liabilities	(1,660)	(1,312)
Total net derivative asset/(liability)	(1,771)	(1,383)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	27	8
Long-term derivative assets	15	13
Current derivative liabilities	(306)	(329)
Long-term derivative liabilities	(22)	(40)
Total net derivative asset/(liability)	(286)	(348)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	18	30
Long-term derivative assets	1	2
Current derivative liabilities	(88)	(87)
Long-term derivative liabilities	(4)	(3)
Total net derivative asset/(liability)	(73)	(58)
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Current derivative liabilities	(10)	(3)
Long-term derivative liabilities	(8)	(1)
Total net derivative asset/(liability)	(18)	(4)
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	82	114
Long-term derivative assets	21	21
Current derivative liabilities	(152)	(339)
Long-term derivative liabilities	(156)	(183)
Total net derivative asset/(liability)	(205)	(387)
Level 3 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	82	114
Long-term derivative assets	21	21
Current derivative liabilities	(152)	(339)
Long-term derivative liabilities	(156)	(183)
Total net derivative asset/(liability)	$ (205)	$ (387)