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Regulatory Assets and Liabilities (Regulated Liabilities) (Details) - CAD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Regulatory Liabilities [Line Items] |  |
Regulatory Liability, Current | $ 226 | $ 362
Regulatory Liability, Noncurrent | 2,242 | 1,277
Regulatory Liabilities, Total | 2,468 | 1,639
Deferred income tax regulatory assets [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory Liabilities, Total | 1,116 | 26
Accumulated reserve - cost of removal [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory Liabilities, Total | 894 | 990
Deferrals related to derivative instruments [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory Liabilities, Total | 182 | 230
Fuel Adjustment Mechanism Liability [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory Liabilities, Total | 177 | 94
Cost Recovery Clauses Liability [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory Liabilities, Total | 51 | 153
Self Insurance Fund [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory Liabilities, Total | 28 | 30
Bill Reduction Credit [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory Liabilities, Total | 4 | 10
Storm Reserve [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory Liabilities, Total | 0 | 75
Other Regulatory Assets (Liabilities) [Member] |  |
Regulatory Liabilities [Line Items] |  |
Regulatory Liabilities, Total | $ 16 | $ 31

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- Definition The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + 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 amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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- Definition The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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