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Pension and other PostRetirement Benefits - Actuarial assumptions (Details) - CAD CAD in Millions | 12 Months Ended |
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Dec. 31, 2017 | Dec. 31, 2016
Pension and other PostRetirement Benefits, |  |
Actual experience period | 5 years |
Increase in actuarial assumption (as a percent) | 1.00% |
Decrease in actuarial assumption (as a percent) | 1.00% |
Defined benefits pension plans |  |
Pension and other PostRetirement Benefits, |  |
Discount rate | 3.40% | 3.90%
Rate of compensation increase | 3.00% | 3.20%
Defined benefits pension plans | Discount rate |  |
Pension and other PostRetirement Benefits, |  |
Increase (decrease) in interest and service cost due to increase in actuarial assumption | CAD (19) |
Increase (decrease) in service and interest cost due to decrease in actuarial assumption | 24 |
Increase (decrease) in net obligation due to increase in actuarial assumption | (859) |
Increase (decrease) in net obligation due to decrease in actuarial assumption | CAD 1,107 |
Defined benefits other post retirement benefits |  |
Pension and other PostRetirement Benefits, |  |
Discount rate | 3.40% | 3.80%
Rate of compensation increase | 3.00% | 3.00%
Assumption of percentage of increase of healthcare costs | 6.50% | 6.50%
Percentage of decrease in healthcare cost in year two | 0.50% |
Assumption of percentage of increase of healthcare costs after 2022 | 5.00% |
Increase (decrease) in net obligation due to increase in actuarial assumption | CAD (72) |
Increase (decrease) in net obligation due to decrease in actuarial assumption | 89 |
Defined benefits other post retirement benefits | Actuarial assumption of medical cost trend rates |  |
Pension and other PostRetirement Benefits, |  |
Increase (decrease) in interest and service cost due to increase in actuarial assumption | 1 |
Increase (decrease) in service and interest cost due to decrease in actuarial assumption | (1) |
Increase (decrease) in net obligation due to increase in actuarial assumption | 30 |
Increase (decrease) in net obligation due to decrease in actuarial assumption | CAD (25) |

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- Definition The discount rate used as the significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 144 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_144&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The expected rate of increases in pension that is used as a significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit plans [member]; Defined benefit obligation, at present value; Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 144 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_144&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition Medical cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation. [Refer: Defined benefit obligation, at present value; Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 144 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_144&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The increase (decrease) in a defined benefit obligation that would have been caused by a decrease in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_145_a&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The increase (decrease) in a defined benefit obligation that would have been caused by an increase in a significant actuarial assumption that was reasonably possible at the end of the reporting period. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_145_a&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The reasonably possible percentage of the decrease in the actuarial assumption used to determine the present value of defined benefit obligation. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_145_a&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition The reasonably possible percentage of the increase in the actuarial assumption used to determine the present value of defined benefit obligation. [Refer: Actuarial assumptions [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2017-01-01 -Paragraph 145 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=19&date=2017-03-01&anchor=para_145_a&doctype=Standard -URIDate 2017-03-09 + Details
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- Definition Actual experience period. + References No definition available. + Details
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- Definition Medical cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation after year five. + References No definition available. + Details
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- Definition The increase (decrease) in interest and service cost that would have been caused by an increase in a significant actuarial assumption that was reasonably possible at the end of the reporting period. + References No definition available. + Details
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- Definition percentage of decrease in healthcare cost in year six + References No definition available. + Details
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