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Risk Management Activities - Foreign Currency Rate Risk Management (Details) - CAD CAD in Millions | 3 Months Ended | 12 Months Ended
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Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | [1] | Dec. 31, 2015 | [1]
Financial Instruments [Abstract] |  |  |  |  |
Derivative hedge, notional amount |  | CAD 690 |  |  |
Net gains on derivatives designated as cash flow hedges, net of tax | CAD 3 | CAD 214 | [1] | CAD 179 | CAD 375
[1] | (7) Net of income tax recovery of 77 for the year ended Dec. 31, 2017 (2016 - 92 expense, 2015 - 138 expense).

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- Definition The gains (losses) recognised in other comprehensive income on cash flow hedges, net of tax. [Refer: Cash flow hedges [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Note Expiry date 2018-01-01 -Name IFRS -Number 7 -IssueDate 2017-01-01 -Paragraph 23 -Subparagraph c -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-02&anchor=para_23_c&doctype=Standard&book=B -URIDate 2017-03-09 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 1 -IssueDate 2017-01-01 -Paragraph 91 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2017-03-01&anchor=para_91_a&doctype=Standard -URIDate 2017-03-09 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Note Effective 2018-01-01 -Name IFRS -Number 7 -IssueDate 2017-01-01 -Paragraph 24C -Subparagraph b -Clause i -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_24C_b_i&doctype=Standard -URIDate 2017-03-09 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Note Effective 2018-01-01 -Name IFRS -Number 7 -IssueDate 2017-01-01 -Paragraph 24E -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2017-03-01&anchor=para_24E_a&doctype=Standard -URIDate 2017-03-09 + Details
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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 Financial Instruments [Abstract] + References No definition available. + Details
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