RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - TOTAL DERIVATIVE INSTRUMENTS (Details) - CAD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	$ (2,283)	$ (2,192)
Derivative liabilities, Total Net Derivative Instruments	(2,283)	(2,192)
Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(2,310)	(1,991)
Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	49	(155)
Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(10)	(46)
Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(12)
Accounts receivable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	236	296
Derivative assets, Amounts Available for Offset	(137)	(150)
Derivative assets, Total Net Derivative Instruments	99	146
Accounts receivable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	185	281
Accounts receivable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	50	9
Accounts receivable and other | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	1	4
Accounts receivable and other | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		2
Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	115	181
Derivative assets, Amounts Available for Offset	(55)	(146)
Derivative assets, Total Net Derivative Instruments	60	35
Deferred amounts and other assets | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	51	149
Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	64	25
Deferred amounts and other assets | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		1
Deferred amounts and other assets | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		6
Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1,034)	(1,130)
Derivative liabilities, Amounts Available for Offset	137	150
Derivative liabilities, Total Net Derivative Instruments	(897)	(980)
Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(969)	(936)
Accounts payable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(56)	(146)
Accounts payable and other | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	0	(42)
Accounts payable and other | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(9)	(6)
Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1,600)	(1,539)
Derivative liabilities, Amounts Available for Offset	55	146
Derivative liabilities, Total Net Derivative Instruments	(1,545)	(1,393)
Other long-term liabilities | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1,577)	(1,485)
Other long-term liabilities | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(9)	(43)
Other long-term liabilities | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(11)	(9)
Other long-term liabilities | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(3)	(2)
Foreign exchange contracts
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(1,697)	(1,383)
Derivative liabilities, Total Net Derivative Instruments	(1,697)	(1,383)
Foreign exchange contracts | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(1,686)	(1,330)
Foreign exchange contracts | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(1)	(7)
Foreign exchange contracts | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(10)	(46)
Foreign exchange contracts | Accounts receivable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	67	143
Derivative assets, Amounts Available for Offset	(49)	(83)
Derivative assets, Total Net Derivative Instruments	18	60
Foreign exchange contracts | Accounts receivable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	66	138
Foreign exchange contracts | Accounts receivable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		1
Foreign exchange contracts | Accounts receivable and other | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	1	4
Foreign exchange contracts | Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	43	145
Derivative assets, Amounts Available for Offset	(29)	(125)
Derivative assets, Total Net Derivative Instruments	14	20
Foreign exchange contracts | Deferred amounts and other assets | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	39	143
Foreign exchange contracts | Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	4	1
Foreign exchange contracts | Deferred amounts and other assets | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		1
Foreign exchange contracts | Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(376)	(359)
Derivative liabilities, Amounts Available for Offset	49	83
Derivative liabilities, Total Net Derivative Instruments	(327)	(276)
Foreign exchange contracts | Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(371)	(312)
Foreign exchange contracts | Accounts payable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(5)	(5)
Foreign exchange contracts | Accounts payable and other | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	0	(42)
Foreign exchange contracts | Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1,431)	(1,312)
Derivative liabilities, Amounts Available for Offset	29	125
Derivative liabilities, Total Net Derivative Instruments	(1,402)	(1,187)
Foreign exchange contracts | Other long-term liabilities | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1,420)	(1,299)
Foreign exchange contracts | Other long-term liabilities | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented		(4)
Foreign exchange contracts | Other long-term liabilities | Net Investment Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(11)	(9)
Interest rate contracts
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(156)	(348)
Derivative liabilities, Total Net Derivative Instruments	(156)	(348)
Interest rate contracts | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(179)	(183)
Interest rate contracts | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	35	(165)
Interest rate contracts | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(12)
Interest rate contracts | Accounts receivable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	49	8
Derivative assets, Amounts Available for Offset	(3)	(3)
Derivative assets, Total Net Derivative Instruments	46	5
Interest rate contracts | Accounts receivable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	49	6
Interest rate contracts | Accounts receivable and other | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		2
Interest rate contracts | Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	42	13
Derivative assets, Amounts Available for Offset	(1)	(2)
Derivative assets, Total Net Derivative Instruments	41	11
Interest rate contracts | Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	42	7
Interest rate contracts | Deferred amounts and other assets | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented		6
Interest rate contracts | Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(238)	(329)
Derivative liabilities, Amounts Available for Offset	3	3
Derivative liabilities, Total Net Derivative Instruments	(235)	(326)
Interest rate contracts | Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(179)	(183)
Interest rate contracts | Accounts payable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(50)	(140)
Interest rate contracts | Accounts payable and other | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(9)	(6)
Interest rate contracts | Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(9)	(40)
Derivative liabilities, Amounts Available for Offset	1	2
Derivative liabilities, Total Net Derivative Instruments	(8)	(38)
Interest rate contracts | Other long-term liabilities | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(6)	(38)
Interest rate contracts | Other long-term liabilities | Fair Value Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(3)	(2)
Commodity contracts
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(414)	(457)
Derivative liabilities, Total Net Derivative Instruments	(414)	(457)
Commodity contracts | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(433)	(476)
Commodity contracts | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	19	19
Commodity contracts | Accounts receivable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	120	145
Derivative assets, Amounts Available for Offset	(85)	(64)
Derivative assets, Total Net Derivative Instruments	35	81
Commodity contracts | Accounts receivable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	119	143
Commodity contracts | Accounts receivable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	1	2
Commodity contracts | Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	30	23
Derivative assets, Amounts Available for Offset	(25)	(19)
Derivative assets, Total Net Derivative Instruments	5	4
Commodity contracts | Deferred amounts and other assets | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	12	6
Commodity contracts | Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	18	17
Commodity contracts | Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(411)	(439)
Derivative liabilities, Amounts Available for Offset	85	64
Derivative liabilities, Total Net Derivative Instruments	(326)	(375)
Commodity contracts | Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(411)	(439)
Commodity contracts | Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(153)	(186)
Derivative liabilities, Amounts Available for Offset	25	19
Derivative liabilities, Total Net Derivative Instruments	(128)	(167)
Commodity contracts | Other long-term liabilities | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(153)	(186)
Other contracts
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(16)	(4)
Derivative liabilities, Total Net Derivative Instruments	(16)	(4)
Other contracts | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(12)	(2)
Other contracts | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Total net derivative asset/(liability)	(4)	(2)
Other contracts | Deferred amounts and other assets
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	0
Derivative assets, Amounts Available for Offset	0
Derivative assets, Total Net Derivative Instruments	0
Other contracts | Deferred amounts and other assets | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative assets, Gross Derivative Instruments as Presented	0
Other contracts | Accounts payable and other
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(9)	(3)
Derivative liabilities, Total Net Derivative Instruments	(9)	(3)
Other contracts | Accounts payable and other | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(8)	(2)
Other contracts | Accounts payable and other | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(1)	(1)
Other contracts | Other long-term liabilities
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(7)	(1)
Derivative liabilities, Amounts Available for Offset	0
Derivative liabilities, Total Net Derivative Instruments	(7)	(1)
Other contracts | Other long-term liabilities | Non-Qualifying Derivative Instruments
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	(4)
Other contracts | Other long-term liabilities | Cash Flow Hedges | Designated as hedging instrument
TOTAL DERIVATIVE INSTRUMENTS
Derivative liabilities, Gross Derivative Instruments as Presented	$ (3)	$ (1)