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Derivatives (Changes in Realized and Unrealized Gains (Losses) with Respect to Derivatives Receiving Regulatory Deferral) (Details) - CAD CAD in Millions | 12 Months Ended
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Dec. 31, 2016 | Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized Gain (Loss) on Derivatives | CAD (258.0) | CAD (96.0)
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Commodity swaps and forwards [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | 163.0 | (30.0)
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Commodity swaps and forwards [Member] | Regulatory Assets [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized Gain (Loss) on Derivatives | 40.0 | (24.0)
Derivative, Gain (Loss) on Derivative, Net | 0.0 | (3.0)
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Commodity swaps and forwards [Member] | Regulatory Liabilities [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized Gain (Loss) on Derivatives | 101.0 | 1.0
Derivative, Gain (Loss) on Derivative, Net | 0.0 | 0.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Commodity swaps and forwards [Member] | Property, Plant and Equipment [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | 0.0 | 0.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Commodity swaps and forwards [Member] | Inventory [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | 5.0 | 12.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Commodity swaps and forwards [Member] | Non-regulated fuel for generation and purchased power [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | 17.0 | (16.0)
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | (2.0) | 2.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Regulatory Assets [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized Gain (Loss) on Derivatives | 0.0 | 0.0
Derivative, Gain (Loss) on Derivative, Net | 0.0 | 0.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Regulatory Liabilities [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized Gain (Loss) on Derivatives | (1.0) | 9.0
Derivative, Gain (Loss) on Derivative, Net | 0.0 | 0.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Property, Plant and Equipment [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | 0.0 | 0.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Inventory [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | 0.0 | 0.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Physical natural gas purchases and sales [Member] | Non-regulated fuel for generation and purchased power [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | (1.0) | (7.0)
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | (90.0) | 103.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] | Regulatory Assets [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized Gain (Loss) on Derivatives | (2.0) | (7.0)
Derivative, Gain (Loss) on Derivative, Net | 12.0 | 0.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] | Regulatory Liabilities [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Unrealized Gain (Loss) on Derivatives | (30.0) | 173.0
Derivative, Gain (Loss) on Derivative, Net | (8.0) | 0.0
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] | Property, Plant and Equipment [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | 0.0 | (1.0)
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] | Inventory [Member] |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |
Derivative, Gain (Loss) on Derivative, Net | (44.0) | (44.0)
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [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 | CAD (18.0) | CAD (18.0)

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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=64837297&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=64837297&loc=SL5624171-113959 + Details
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- Definition 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. + References No definition available. + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585 + Details
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