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Fair Values (Quantitative Information Significant Unobservable Inputs - Assets & Liabilities) (Details) - Designated as Hedging Instrument [Member] - CAD CAD / shares in Units, CAD in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015
Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | CAD 40.0 | CAD 51.0
Financial and Nonfinancial Liabilities, Fair Value Disclosure | 389.0 | 277.0
Derivative Assets (Liabilities), at Fair Value, Net, Total | (349.0) | (226.0)
Held for Trading Hedge [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure |  | 54.0
Financial and Nonfinancial Liabilities, Fair Value Disclosure | 0.0 |
Held for Trading Hedge [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Financial and Nonfinancial Liabilities, Fair Value Disclosure | CAD 3.0 |
Held for Trading Hedge [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure |  | CAD 3.0
Held for Trading Hedge [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes |  | CAD 1.25
Fair Value Inputs, Comparability Adjustments |  | (0.06%)
Fair Value Inputs, Probability of Default |  | 0.00%
Fair Value Inputs, Discount Rate |  | 0.00%
Held for Trading Hedge [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes |  | CAD 15.74
Fair Value Inputs, Comparability Adjustments |  | 0.95%
Fair Value Inputs, Probability of Default |  | 0.09%
Fair Value Inputs, Discount Rate |  | 0.08%
Held for Trading Hedge [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes |  | CAD 6.19
Fair Value Inputs, Comparability Adjustments |  | 0.68%
Fair Value Inputs, Probability of Default |  | 0.00%
Fair Value Inputs, Discount Rate |  | 0.00%
Held for Trading Hedge [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 1.83 | CAD 46.45
Fair Value Inputs, Comparability Adjustments | (0.11%) |
Fair Value Inputs, Probability of Default | 0.00% |
Fair Value Inputs, Entity Credit Risk |  | 0.06%
Fair Value Inputs, Discount Rate | 0.00% | 0.00%
Held for Trading Hedge [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 11.87 | CAD 103.27
Fair Value Inputs, Comparability Adjustments | 0.64% |
Fair Value Inputs, Probability of Default | 0.05% |
Fair Value Inputs, Entity Credit Risk |  | 0.06%
Fair Value Inputs, Discount Rate | 0.10% | 4.15%
Held for Trading Hedge [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 5.93 | CAD 70.69
Fair Value Inputs, Comparability Adjustments | 0.27% |
Fair Value Inputs, Probability of Default | 0.01% |
Fair Value Inputs, Entity Credit Risk |  | 0.06%
Fair Value Inputs, Discount Rate | 0.01% | 0.47%
Held for Trading Hedge [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes |  | CAD 1.13
Fair Value Inputs, Probability of Default |  | 0.00%
Fair Value Inputs, Discount Rate |  | 0.00%
Held for Trading Hedge [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes |  | CAD 9.12
Fair Value Inputs, Probability of Default |  | 0.10%
Fair Value Inputs, Discount Rate |  | 0.33%
Held for Trading Hedge [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes |  | CAD 3.26
Fair Value Inputs, Probability of Default |  | 0.01%
Fair Value Inputs, Discount Rate |  | 0.04%
Held for Trading Hedge [Member] | Physical natural gas purchases and sales [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | CAD 12.0 |
Held for Trading Hedge [Member] | Physical natural gas purchases and sales [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Probability of Default |  | 0.01%
Held for Trading Hedge [Member] | Physical natural gas purchases and sales [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 1.83 | CAD 5.15
Fair Value Inputs, Comparability Adjustments | (0.11%) |
Fair Value Inputs, Probability of Default | 0.00% |
Fair Value Inputs, Discount Rate | 0.00% |
Held for Trading Hedge [Member] | Physical natural gas purchases and sales [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 11.87 | 6.21
Fair Value Inputs, Comparability Adjustments | 0.64% |
Fair Value Inputs, Probability of Default | 0.05% |
Fair Value Inputs, Discount Rate | 0.10% |
Held for Trading Hedge [Member] | Physical natural gas purchases and sales [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 6.16 | CAD 5.72
Fair Value Inputs, Comparability Adjustments | 0.39% |
Fair Value Inputs, Probability of Default | 0.00% | 0.01%
Fair Value Inputs, Discount Rate | 0.00% |
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | CAD 0.0 | CAD (8.0)
Financial and Nonfinancial Liabilities, Fair Value Disclosure | CAD 0.0 | CAD (2.0)
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 19.7 | CAD 26.27
Fair Value Inputs, Correlation factor |  | 0.98%
Fair Value Inputs, Entity Credit Risk | 0.00% | 0.00%
Fair Value Inputs, Discount Rate | 0.09% | 0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 76.94 | CAD 129.2
Fair Value Inputs, Correlation factor |  | 1.00%
Fair Value Inputs, Entity Credit Risk | 0.02% | 0.02%
Fair Value Inputs, Discount Rate | 0.02% | 0.15%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 33.48 | CAD 70.82
Fair Value Inputs, Correlation factor |  | 0.99%
Fair Value Inputs, Entity Credit Risk | 0.01% | 0.00%
Fair Value Inputs, Discount Rate | 0.03% | 0.01%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 19.7 | CAD 26.27
Fair Value Inputs, Correlation factor |  | 0.98%
Fair Value Inputs, Probability of Default | 0.00% | 0.00%
Fair Value Inputs, Discount Rate | 0.02% | 0.00%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 76.94 | CAD 129.2
Fair Value Inputs, Correlation factor |  | 1.00%
Fair Value Inputs, Probability of Default | 0.01% | 0.02%
Fair Value Inputs, Discount Rate | 0.09% | 0.15%
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 33.37 | CAD 70.45
Fair Value Inputs, Correlation factor |  | 0.99%
Fair Value Inputs, Probability of Default | 0.00% | 0.00%
Fair Value Inputs, Discount Rate | 0.03% | 0.01%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | CAD 27.0 |
Financial and Nonfinancial Liabilities, Fair Value Disclosure | CAD 386.0 | CAD 279.0
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Entity Credit Risk |  | 0.00%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 1.55 | CAD 0.74
Fair Value Inputs, Probability of Default |  | 0.00%
Fair Value Inputs, Entity Credit Risk | 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] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 11.87 | CAD 10.59
Fair Value Inputs, Probability of Default |  | 0.03%
Fair Value Inputs, Entity Credit Risk | 0.70% |
Fair Value Inputs, Discount Rate | 0.14% | 0.07%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 6.26 | CAD 5.58
Fair Value Inputs, Probability of Default |  | 0.00%
Fair Value Inputs, Entity Credit Risk | 0.00% | 0.00%
Fair Value Inputs, Discount Rate | 0.02% | 0.01%
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 1.41 |
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] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 11.87 |
Fair Value Inputs, Probability of Default | 7.00% |
Fair Value Inputs, Discount Rate | 0.32% |
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 3.87 | CAD 5.56
Fair Value Inputs, Probability of Default | 0.01% | 0.46%
Fair Value Inputs, Discount Rate | 0.05% | 5.28%
Regulatory Deferral Hedge [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Financial and Nonfinancial Liabilities, Fair Value Disclosure | CAD 0.0 |
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | 1.0 |
Financial and Nonfinancial Liabilities, Fair Value Disclosure | CAD 0.0 |
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Probability of Default | 0.80% |
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 0 |
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | 69.64 |
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 69.64 |
Fair Value Inputs, Probability of Default | 0.80% |
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Fair Value, Inputs, Level 3 [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | CAD 0.0 | CAD 2.0
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Probability of Default | 0.01% |
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Minimum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 4.75 |
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Maximum [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | 4.94 |
Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Market Approach Valuation Technique [Member] | Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member] | Weighted Average [Member] |  |
Fair Value Inputs [Abstract] |  |
Fair Value Inputs, Offered Quotes | CAD 4.87 |
Fair Value Inputs, Probability of Default | 0.01% |
Cash Flow Hedging [Member] | Power swaps and physical contracts [Member] |  |
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items] |  |
Assets, Fair Value Disclosure | CAD 0.0 |

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- Definition 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 + Details
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- Definition 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. + References 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 + Details
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- Definition 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. + 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 (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition 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. + 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 (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition 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 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Price at which a dealer is willing to buy an instrument, 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 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Percentage of likelihood a loan will not be repaid and instead default, 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 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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