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RISK MANAGEMENT AND FINANCIAL INSTRUMENTS - Unrealized and Realized (Losses)/Gains (Details) - CAD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
U.S. Northeast Merchant Power Assets
Derivative [Line Items]
Gain (loss) on cash flow hedge	$ 0	$ 0	$ 0
Commodities
Derivative [Line Items]
Amount of unrealized gains/(losses) in the year	(111,000,000)	28,000,000	62,000,000
Amount of realized gains/(losses) in the year	378,000,000	351,000,000	(107,000,000)
Commodities | Derivative instruments in hedging relationships
Derivative [Line Items]
Amount of realized (losses)/gains in the year	(6,000,000)	(1,000,000)	23,000,000
Foreign exchange
Derivative [Line Items]
Amount of unrealized gains/(losses) in the year	245,000,000	(248,000,000)	88,000,000
Amount of realized gains/(losses) in the year	(70,000,000)	(24,000,000)	18,000,000
Foreign exchange | Derivative instruments in hedging relationships
Derivative [Line Items]
Amount of realized (losses)/gains in the year	0	0	5,000,000
Interest rate
Derivative [Line Items]
Amount of unrealized gains/(losses) in the year	0	0	(1,000,000)
Amount of realized gains/(losses) in the year	0	0	1,000,000
Interest rate | Derivative instruments in hedging relationships
Derivative [Line Items]
Amount of realized (losses)/gains in the year	$ 2,000,000	$ (1,000,000)	$ 1,000,000

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- Definition

Realized Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net

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- Definition

Unrealized Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net

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- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

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- Definition

The amount of net gain (loss) reclassified into earnings in the period when cash flow hedge is discontinued because it is probable that the original forecasted transactions will not occur by the end of the original period or an additional two month time period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=112277219&loc=d3e80748-113994

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