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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 |

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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 + Details
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- Definition 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 + Details
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