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Accumulated Other Comprehensive Income (Reclassifications Out of Accumulated Other Comprehensive Income (Loss)) (Details) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Non-regulated fuel for generation and purchased power | $ 209 | $ 313
Income (Loss) from Equity Method Investments | 124 | 100
Operating, maintenance and general | 1,399 | 1,137
Other Income (Expense) | 2 | 174
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest | 819 | 244
Income tax expense (recovery) | 520 | (22)
Net income | 299 | 266
Reclassification out of Accumulated Other Comprehensive Income [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Net income | 51 | 65
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest | 7 | 11
Income tax expense (recovery) | 1 | 0
Net income | 8 | 11
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] | Power and Gas Swaps [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Non-regulated fuel for generation and purchased power | (3) | (2)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] | Interest Rate Swap [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Income (Loss) from Equity Method Investments | 0 | 1
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member] | Foreign Exchange Forward [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Regulated | 10 | 12
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest | 43 | 15
Income tax expense (recovery) | 1 | (3)
Net income | 44 | 12
Reclassification out of Accumulated Other Comprehensive Income [Member] | Defined Benefit Plans Adjustment, Net Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Operating, maintenance and general | 40 | 41
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Including Portion Attributable to Noncontrolling Interest [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Operating, maintenance and general | (8) | (9)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment, Net Transition Including Portion Attributable to Noncontrolling Interest [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Pension and Other Postretirement Benefit Expense | 11 | (17)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Available For Sale Securities Gain Loss Including Portion Attributable To Noncontrolling Interest [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Other Income (Expense) | (1) | (4)
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest | (1) | (4)
Income tax expense (recovery) | 0 | 0
Net income | (1) | (4)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member] |  |
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] |  |
Equity Method Investment, Realized Gain (Loss) on Disposal | 0 | 54
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest | 0 | 54
Income tax expense (recovery) | 0 | (8)
Net income | $ 0 | $ 46

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