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Fair Values (Quantitative Information Significant Unobservable Inputs - Assets & Liabilities) (Details) - Designated as Hedging Instrument [Member] - Fair Value, Inputs, Level 3 [Member] - CAD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative Financial Instruments, Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 183	$ 389
Derivative Assets (Liabilities), at Fair Value, Net, Total	(149)	(349)
Derivative Financial Instruments, Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	34	40
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ (6)
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 24.88
Fair Value Inputs, Entity Credit Risk	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 117.9
Fair Value Inputs, Entity Credit Risk	0.01%
Fair Value Inputs, Discount Rate	0.13%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Liabilities [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 95.46
Fair Value Inputs, Entity Credit Risk	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 1
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 24.88
Fair Value Inputs, Probability of Default	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 117.9
Fair Value Inputs, Probability of Default	0.00%
Fair Value Inputs, Discount Rate	0.13%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 92.93
Fair Value Inputs, Probability of Default	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Modelled pricing with correlation factor [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 2
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Modelled pricing with correlation factor [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 94.5
Fair Value Inputs, Correlation factor	0.94%
Fair Value Inputs, Probability of Default	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Modelled pricing with correlation factor [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 117
Fair Value Inputs, Correlation factor	0.99%
Fair Value Inputs, Probability of Default	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Modelled pricing with correlation factor [Member] | Derivative Financial Instruments, Liabilities [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 105.52
Fair Value Inputs, Correlation factor	0.96%
Fair Value Inputs, Probability of Default	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Modelled pricing with correlation factor [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 8
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Modelled pricing with correlation factor [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 63.48
Fair Value Inputs, Correlation factor	0.94%
Fair Value Inputs, Probability of Default	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Modelled pricing with correlation factor [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 117
Fair Value Inputs, Correlation factor	0.99%
Fair Value Inputs, Probability of Default	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Modelled pricing with correlation factor [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 102.68
Fair Value Inputs, Correlation factor	0.96%
Fair Value Inputs, Probability of Default	0.00%
Fair Value Inputs, Discount Rate	0.00%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 172	$ 386
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 1.89	$ 1.55
Fair Value Inputs, Entity Credit Risk	0.00%	0.00%
Fair Value Inputs, Discount Rate	0.00%	0.00%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 11.81	$ 11.87
Fair Value Inputs, Entity Credit Risk	0.00%	0.07%
Fair Value Inputs, Discount Rate	0.12%	0.14%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Liabilities [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 4.64	$ 6.26
Fair Value Inputs, Entity Credit Risk	0.00%	0.00%
Fair Value Inputs, Discount Rate	0.02%	0.02%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 18	$ 27
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 2.06	$ 1.41
Fair Value Inputs, Probability of Default	0.00%	0.00%
Fair Value Inputs, Discount Rate	0.00%	0.00%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 8.24	$ 11.87
Fair Value Inputs, Probability of Default	0.05%	0.07%
Fair Value Inputs, Discount Rate	0.29%	0.32%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 3.61	$ 3.87
Fair Value Inputs, Probability of Default	0.00%	0.01%
Fair Value Inputs, Discount Rate	0.06%	0.05%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Modelled pricing with basis adjustments [Member] | Derivative Financial Instruments, Liabilities [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 15	$ 3
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Modelled pricing with basis adjustments [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 2.15	$ 1.83
Fair Value Inputs, Comparability Adjustments	0.08%	(0.11%)
Fair Value Inputs, Entity Credit Risk	0.00%	0.00%
Fair Value Inputs, Discount Rate	0.00%	0.00%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Modelled pricing with basis adjustments [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 12.52	$ 11.87
Fair Value Inputs, Comparability Adjustments	0.71%	0.64%
Fair Value Inputs, Entity Credit Risk	0.00%	0.05%
Fair Value Inputs, Discount Rate	0.08%	0.10%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Modelled pricing with basis adjustments [Member] | Derivative Financial Instruments, Liabilities [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 8.94	$ 5.93
Fair Value Inputs, Comparability Adjustments	0.53%	0.27%
Fair Value Inputs, Entity Credit Risk	0.00%	0.01%
Fair Value Inputs, Discount Rate	0.01%	0.01%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Modelled pricing with basis adjustments [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 7	$ 12
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Modelled pricing with basis adjustments [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 2.04	$ 1.83
Fair Value Inputs, Comparability Adjustments	0.08%	(0.11%)
Fair Value Inputs, Probability of Default	0.00%	0.00%
Fair Value Inputs, Discount Rate	0.00%	0.00%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Modelled pricing with basis adjustments [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 12.52	$ 11.87
Fair Value Inputs, Comparability Adjustments	0.71%	0.64%
Fair Value Inputs, Probability of Default	0.00%	0.05%
Fair Value Inputs, Discount Rate	0.09%	0.10%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Modelled pricing with basis adjustments [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes	$ 6.42	$ 6.16
Fair Value Inputs, Comparability Adjustments	0.52%	0.39%
Fair Value Inputs, Probability of Default	0.00%	0.00%
Fair Value Inputs, Discount Rate	0.01%	0.00%
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member]
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Assets, Fair Value Disclosure		$ 1
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Market Approach Valuation Technique [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member]
Fair Value Inputs [Abstract]
Fair Value Inputs, Offered Quotes		$ 69.64
Fair Value Inputs, Probability of Default		0.80%

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ 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 (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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Percentage of adjustment to the observed price for a similar asset or liability when an identical transaction does not exist, used as an input to measure fair value.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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-Section 50
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Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Risk associated with risk of default, credit spread risk and downgrade risk, expressed as a percentage, used as an input to measure fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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Price at which a dealer is willing to buy an instrument, used as an input to measure fair value.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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-Section 50
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Percentage of likelihood a loan will not be repaid and instead default, used as an input to measure fair value.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Fair value of financial and nonfinancial obligations.

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