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Derivatives (Derivative Assets and Liabilities) (Details) - CAD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | $ 434 | $ 407
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (181) | (131)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 491 | 606
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | (181) | (131)
Current Derivative Assets or Liabilities [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Total | 141 | 145
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Total | 227 | 325
Long term derivative assets or liabilities [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Total | 112 | 131
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Total | 83 | 150
Cash Flow Hedging [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 7 | 10
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 7 | 27
Cash Flow Hedging [Member] | Power swaps [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 5 | 10
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 2 | 5
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 2 | 0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 5 | 22
Regulatory Deferral Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 195 | 239
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 28 | 22
Regulatory Deferral Hedge [Member] | Power swaps [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 5 | 7
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 3 | 4
Regulatory Deferral Hedge [Member] | Foreign Exchange Forward [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 32 | 106
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 4 | 0
Regulatory Deferral Hedge [Member] | Derivative instrument whose primary underlying risk is tied to commodity prices for coal purchases. |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 137 | 83
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 10 | 9
Regulatory Deferral Hedge [Member] | Natural gas purchases and sales [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 6 | 33
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 7 | 2
Regulatory Deferral Hedge [Member] | Heavy fuel oil purchases [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 15 | 10
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 4 | 7
Held for Trading Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 230 | 158
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 456 | 555
Held for Trading Hedge [Member] | Power swaps and physical contracts [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 125 | 47
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 162 | 71
Held for Trading Hedge [Member] | Natural gas swaps, futures, forwards, physical contracts [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 105 | 111
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 294 | 484
Other Derivatives Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 2 | 0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0 | 2
Other Derivatives Hedge [Member] | Foreign Exchange Forward [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 0 | 0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0 | 1
Other Derivatives Hedge [Member] | Interest Rate Swap [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Fair Value, Gross Asset | 2 | 0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Fair Value, Gross Liability | 0 | 1
Designated as Hedging Instrument [Member] | Regulatory Deferral Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (14) | (10)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | (14) | (10)
Designated as Hedging Instrument [Member] | Held for Trading Hedge [Member] |  |
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Asset, Collateral, Obligation to Return Cash, Offset | (167) | (121)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral [Abstract] |  |
Derivative Liability, Collateral, Right to Reclaim Cash, Offset | $ (167) | $ (121)

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