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Fair Values (fair value of the Level 3 financial assets) (Details) - Designated as Hedging Instrument [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017 CAD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 40
Total realized and unrealized gains (losses) included in non-regulated operating revenues	10
Net transfers out of Level 3	15
Ending Balance	34
Regulated fuel for generation and purchased power [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Increase (reduction) in benefit included in regulated anmd non-gulated fuel for generation and purchased power	(1)
Regulatory Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Unrealized gains (losses) included in regulatory assets or liabilities	0
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0
Total realized and unrealized gains (losses) included in non-regulated operating revenues	0
Net transfers out of Level 3	0
Ending Balance	0
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Regulated fuel for generation and purchased power [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Increase (reduction) in benefit included in regulated anmd non-gulated fuel for generation and purchased power	(1)
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Regulatory Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Unrealized gains (losses) included in regulatory assets or liabilities	1
Regulatory Deferral Hedge [Member] | Oil Financial Derivatives [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	1
Total realized and unrealized gains (losses) included in non-regulated operating revenues	0
Net transfers out of Level 3	0
Ending Balance	0
Regulatory Deferral Hedge [Member] | Oil Financial Derivatives [Member] | Regulatory Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Increase (reduction) in benefit included in regulated anmd non-gulated fuel for generation and purchased power	0
Unrealized gains (losses) included in regulatory assets or liabilities	(1)
Cash Flow Hedging And Held for Trading Hedge [Member] | Natural Gas [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	39
Total realized and unrealized gains (losses) included in non-regulated operating revenues	1
Net transfers out of Level 3	15
Ending Balance	25
Cash Flow Hedging And Held for Trading Hedge [Member] | Natural Gas [Member] | Regulated fuel for generation and purchased power [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Increase (reduction) in benefit included in regulated anmd non-gulated fuel for generation and purchased power	0
Cash Flow Hedging And Held for Trading Hedge [Member] | Natural Gas [Member] | Regulatory Assets [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Unrealized gains (losses) included in regulatory assets or liabilities	0
Cash Flow Hedging And Held for Trading Hedge [Member] | Power [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0
Total realized and unrealized gains (losses) included in non-regulated operating revenues	9
Net transfers out of Level 3	0
Ending Balance	9
Cash Flow Hedging And Held for Trading Hedge [Member] | Power [Member] | Regulated fuel for generation and purchased power [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Increase (reduction) in benefit included in regulated anmd non-gulated fuel for generation and purchased power	0
Cash Flow Hedging And Held for Trading Hedge [Member] | Power [Member] | Regulatory Assets [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Unrealized gains (losses) included in regulatory assets or liabilities	$ 0

X

- Definition

Total amount of unrealized (holding) gain (loss) and realized gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

+ References

No definition available.

+ Details

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X

- Definition

Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

+ References

No definition available.

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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