RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Derivative Assets and Liabilities Measured on a Recurring Basis (Details) - CAD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair Value Hierarchy
Derivative Instrument Assets	$ 197	$ 798
Derivative Instrument Liabilities	(196)	(964)
Recurring basis
Fair Value Hierarchy
Assets and (liabilities), net	1	(166)
Recurring basis \| Commodities \| Power
Fair Value Hierarchy
Derivative Instrument Assets	118	768
Derivative Instrument Liabilities	(125)	(654)
Recurring basis \| Foreign exchange
Fair Value Hierarchy
Derivative Instrument Assets	76	18
Derivative Instrument Liabilities	(5)	(295)
Recurring basis \| Interest rate
Fair Value Hierarchy
Derivative Instrument Assets	3	12
Derivative Instrument Liabilities	(66)	(15)
Recurring basis \| Quoted Prices in Active Markets (Level I)
Fair Value Hierarchy
Assets and (liabilities), net	4	26
Recurring basis \| Quoted Prices in Active Markets (Level I) \| Commodities \| Power
Fair Value Hierarchy
Derivative Instrument Assets	81	581
Derivative Instrument Liabilities	(77)	(555)
Recurring basis \| Quoted Prices in Active Markets (Level I) \| Foreign exchange
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	4	(188)
Recurring basis \| Significant Other Observable Inputs (Level II) \| Commodities \| Power
Fair Value Hierarchy
Derivative Instrument Assets	37	187
Derivative Instrument Liabilities	(41)	(95)
Recurring basis \| Significant Other Observable Inputs (Level II) \| Foreign exchange
Fair Value Hierarchy
Derivative Instrument Assets	76	18
Derivative Instrument Liabilities	(5)	(295)
Recurring basis \| Significant Other Observable Inputs (Level II) \| Interest rate
Fair Value Hierarchy
Derivative Instrument Assets	3	12
Derivative Instrument Liabilities	(66)	(15)
Recurring basis \| Significant Unobservable Inputs (Level III)
Fair Value Hierarchy
Assets and (liabilities), net	(7)	(4)
Recurring basis \| Significant Unobservable Inputs (Level III) \| Commodities \| Power
Fair Value Hierarchy
Derivative Instrument Assets	0	0
Derivative Instrument Liabilities	(7)	(4)
Recurring basis \| Significant Unobservable Inputs (Level III) \| Foreign exchange
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	$ 0	$ 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.

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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.

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