RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Derivative Assets and Liabilities Measured on a Recurring Basis (Details) - CAD CAD in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value Hierarchy
Derivative Instrument Assets:	CAD 405	CAD 509
Derivative Instrument Liabilities:	(459)	(937)
Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	319	479
Derivative Instrument Liabilities:	(242)	(448)
Foreign exchange contracts
Fair Value Hierarchy
Derivative Instrument Assets:	78	26
Derivative Instrument Liabilities:	(212)	(486)
Interest rate
Fair Value Hierarchy
Derivative Instrument Assets:	8	4
Derivative Instrument Liabilities:	(5)	(3)
Recurring basis
Fair Value Hierarchy
Assets and (liabilities), net	(54)	(428)
Recurring basis \| Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	319	479
Derivative Instrument Liabilities:	(242)	(448)
Recurring basis \| Foreign exchange contracts
Fair Value Hierarchy
Derivative Instrument Assets:	78	26
Derivative Instrument Liabilities:	(212)	(486)
Recurring basis \| Interest rate
Fair Value Hierarchy
Derivative Instrument Assets:	8	4
Derivative Instrument Liabilities:	(5)	(3)
Recurring basis \| Quoted Prices in Active Markets (Level I)
Fair Value Hierarchy
Assets and (liabilities), net	(6)	32
Recurring basis \| Quoted Prices in Active Markets (Level I) \| Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	21	134
Derivative Instrument Liabilities:	(27)	(102)
Recurring basis \| Quoted Prices in Active Markets (Level I) \| Foreign exchange contracts
Fair Value Hierarchy
Derivative Instrument Assets:	0	0
Derivative Instrument Liabilities:	0	0
Recurring basis \| Quoted Prices in Active Markets (Level I) \| Interest rate
Fair Value Hierarchy
Derivative Instrument Assets:	0	0
Derivative Instrument Liabilities:	0	0
Recurring basis \| Significant Other Observable Inputs (Level II)
Fair Value Hierarchy
Assets and (liabilities), net	(41)	(476)
Recurring basis \| Significant Other Observable Inputs (Level II) \| Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	283	326
Derivative Instrument Liabilities:	(193)	(343)
Recurring basis \| Significant Other Observable Inputs (Level II) \| Foreign exchange contracts
Fair Value Hierarchy
Derivative Instrument Assets:	78	26
Derivative Instrument Liabilities:	(212)	(486)
Recurring basis \| Significant Other Observable Inputs (Level II) \| Interest rate
Fair Value Hierarchy
Derivative Instrument Assets:	8	4
Derivative Instrument Liabilities:	(5)	(3)
Recurring basis \| Significant Unobservable Inputs (Level III)
Fair Value Hierarchy
Assets and (liabilities), net	(7)	16
Recurring basis \| Significant Unobservable Inputs (Level III) \| Commodity contract \| Power
Fair Value Hierarchy
Derivative Instrument Assets:	15	19
Derivative Instrument Liabilities:	(22)	(3)
Recurring basis \| Significant Unobservable Inputs (Level III) \| Foreign exchange contracts
Fair Value Hierarchy
Derivative Instrument Assets:	0	0
Derivative Instrument Liabilities:	0	0
Recurring basis \| Significant Unobservable Inputs (Level III) \| Interest rate
Fair Value Hierarchy
Derivative Instrument Assets:	0	0
Derivative Instrument Liabilities:	CAD 0	CAD 0

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-Section 50
-Paragraph 4B
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-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

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