RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - FAIR VALUE OF DERIVATIVES (Details) - CAD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value of Derivatives
Total net derivative asset/(liability)	$ (2,283)	$ (2,192)
Foreign exchange contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(1,697)	(1,383)
Interest rate contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(156)	(348)
Commodity contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(414)	(457)
Other contracts
Fair Value of Derivatives
Total net derivative asset/(liability)	(16)	(4)
Recurring basis
Fair Value of Derivatives
Current derivative assets	236	296
Long-term derivative assets	115	181
Current derivative liabilities	(1,034)	(1,130)
Long-term derivative liabilities	(1,600)	(1,539)
Total net derivative asset/(liability)	(2,283)	(2,192)
Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	67	143
Long-term derivative assets	43	145
Current derivative liabilities	(376)	(359)
Long-term derivative liabilities	(1,431)	(1,312)
Total net derivative asset/(liability)	(1,697)	(1,383)
Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	49	8
Long-term derivative assets	42	13
Current derivative liabilities	(238)	(329)
Long-term derivative liabilities	(9)	(40)
Total net derivative asset/(liability)	(156)	(348)
Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	120	145
Long-term derivative assets	30	23
Current derivative liabilities	(411)	(439)
Long-term derivative liabilities	(153)	(186)
Total net derivative asset/(liability)	(414)	(457)
Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	0
Current derivative liabilities	(9)	(3)
Long-term derivative liabilities	(7)	(1)
Total net derivative asset/(liability)	(16)	(4)
Level 1 | Recurring basis
Fair Value of Derivatives
Current derivative assets	1	1
Current derivative liabilities	(11)	(13)
Total net derivative asset/(liability)	(10)	(12)
Level 1 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	1	1
Current derivative liabilities	(11)	(13)
Total net derivative asset/(liability)	(10)	(12)
Level 2 | Recurring basis
Fair Value of Derivatives
Current derivative assets	125	181
Long-term derivative assets	90	160
Current derivative liabilities	(660)	(778)
Long-term derivative liabilities	(1,458)	(1,356)
Total net derivative asset/(liability)	(1,903)	(1,793)
Level 2 | Recurring basis | Foreign exchange contracts
Fair Value of Derivatives
Current derivative assets	67	143
Long-term derivative assets	43	145
Current derivative liabilities	(376)	(359)
Long-term derivative liabilities	(1,431)	(1,312)
Total net derivative asset/(liability)	(1,697)	(1,383)
Level 2 | Recurring basis | Interest rate contracts
Fair Value of Derivatives
Current derivative assets	49	8
Long-term derivative assets	42	13
Current derivative liabilities	(238)	(329)
Long-term derivative liabilities	(9)	(40)
Total net derivative asset/(liability)	(156)	(348)
Level 2 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	9	30
Long-term derivative assets	5	2
Current derivative liabilities	(37)	(87)
Long-term derivative liabilities	(11)	(3)
Total net derivative asset/(liability)	(34)	(58)
Level 2 | Recurring basis | Other contracts
Fair Value of Derivatives
Long-term derivative assets	0
Current derivative liabilities	(9)	(3)
Long-term derivative liabilities	(7)	(1)
Total net derivative asset/(liability)	(16)	(4)
Level 3 | Recurring basis
Fair Value of Derivatives
Current derivative assets	110	114
Long-term derivative assets	25	21
Current derivative liabilities	(363)	(339)
Long-term derivative liabilities	(142)	(183)
Total net derivative asset/(liability)	(370)	(387)
Level 3 | Recurring basis | Commodity contracts
Fair Value of Derivatives
Current derivative assets	110	114
Long-term derivative assets	25	21
Current derivative liabilities	(363)	(339)
Long-term derivative liabilities	(142)	(183)
Total net derivative asset/(liability)	$ (370)	$ (387)