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Derivatives (Realized and Unrealized Gains (Losses) with Respect to HFT Derivatives) (Details) - Designated as Hedging Instrument [Member] - Held for Trading Hedge [Member] - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net	$ 420	$ 59
Power swaps and physical contracts [Member] | Non-regulated fuel for generation and purchased power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net	2	0
Power swaps and physical contracts [Member] | Other Operating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net	7	(1)
Natural gas swaps, futures, forwards, physical contracts [Member] | Non-regulated fuel for generation and purchased power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net	10	(7)
Natural gas swaps, futures, forwards, physical contracts [Member] | Other Operating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net	401	69
Foreign Exchange Option [Member] | Other Operating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net	$ 0	$ (2)

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- Definition

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+ References

No definition available.

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- Definition

Aggregate net gain and losses (whether realized or unrealized) on all derivative instruments held for trading purposes whether or not settled physically.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41322-113958

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4F
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624186-113959

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