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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, 2017 CAD ($)	Dec. 31, 2016 CAD ($)	Dec. 31, 2017 USD ($)
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	$ 5.0
Cash Flow Hedging [Member] | Power swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	3.0	$ 2.0
Unrealized Gain (Loss) on Cash Flow Hedging Instruments, Effective Portion	0.0	2.0
Cash Flow Hedging [Member] | Power swaps [Member] | Non-regulated fuel for generation and purchased power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	3.0	2.0
Cash Flow Hedging [Member] | Power swaps [Member] | Regulated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Power swaps [Member] | GainLossOnInvestmentsMember1
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Power swaps [Member] | Other Operating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Power swaps [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	(1.0)
Unrealized Gain (Loss) on Cash Flow Hedging Instruments, Effective Portion	0.0	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]
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	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	0.0
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | GainLossOnInvestmentsMember1
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	(1.0)
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Other Operating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Interest Rate Swap [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(10.0)	(12.0)
Unrealized Gain (Loss) on Cash Flow Hedging Instruments, Effective Portion	(3.0)	(22.0)
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Non-regulated fuel for generation and purchased power [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Cash Flow Hedging Instruments	0.0	0.0
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Regulated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	(10.0)	(12.0)
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | GainLossOnInvestmentsMember1
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Other Operating Income (Expense) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Gain (Loss) on Derivative, Net	0.0	0.0
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Sales [Member] | 2018 [Member}
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Notional Amount			$ 45
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
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Sales [Member] | 2022 [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	$ 2.0	$ 2.0

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

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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

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