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Derivatives (Derivative Cash Flow Hedges Recorded in Income and AOCI and Notional Volumes of Outstanding Derivatives Designated as Cash Flow Hedges) (Details) $ in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 CAD ($)	Dec. 31, 2017 CAD ($)	Dec. 31, 2018 USD ($)
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	$ 4
Cash Flow Hedging [Member] | Power swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	1	$ 3
Unrealized Gain (Loss) on Cash Flow Hedging Instruments, Effective Portion	(1)	0
Cash Flow Hedging [Member] | Power swaps [Member] | Non-regulated fuel for generation and purchased power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	1	3
Cash Flow Hedging [Member] | Power swaps [Member] | Regulated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0	0
Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0	0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments, Effective Portion	0	0
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Non-regulated fuel for generation and purchased power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0	0
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Regulated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0	0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	5	(10)
Unrealized Gain (Loss) on Cash Flow Hedging Instruments, Effective Portion	(6)	(3)
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Non-regulated fuel for generation and purchased power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0	0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Regulated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	5	(10)
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Sales [Member] | 2019 [Member}
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount			$ 30
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Sales [Member] | 2020 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount			30
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Sales [Member] | 2021 [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount			$ 0
Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	$ (1)	$ 2

X

- Definition

Amount of unrealized gain (loss) related to derivatives designated as cash flow hedging instruments, recorded in results of operations to the extent that the cash flow hedge is determined to be effective.

+ References

No definition available.

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- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

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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/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a),(c),(d),(e)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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