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Pension and Non-pension Post-employment Benefit Plans - Percentages and Assumptions Used in Measuring the Plans (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | ---
Pension Plans |  |  |
Weighted average discount rate at December 31 (i) for: |  |  |
Benefit obligations | 2.50% | 2.60% | 3.80%
Net pension cost | 2.60% | 3.80% | 3.70%
Weighted average rate of compensation increase for: |  |  |
Benefit obligations | 4.00% | 3.90% | 3.80%
Net pension cost | 3.90% | 3.80% | 3.80%
Pension Plans | Discount rate |  |  |
Healthcare cost trend rates: |  |  |
1% Increase | $ (58.9) |  |
1% Decrease | $ 77.4 |  |
Impact on defined benefit obligation, increase in assumption | 1.00% |  |
Impact on defined benefit obligation, decrease in assumption | 1.00% |  |
Pension Plans | Healthcare cost trend rate |  |  |
Healthcare cost trend rates: |  |  |
1% Increase | $ 0.0 |  |
1% Decrease | $ 0.0 |  |
Impact on defined benefit obligation, increase in assumption | 1.00% |  |
Impact on defined benefit obligation, decrease in assumption | 1.00% |  |
Other Benefit Plans |  |  |
Weighted average discount rate at December 31 (i) for: |  |  |
Benefit obligations | 3.60% | 3.90% | 4.10%
Net pension cost | 3.90% | 4.10% | 3.90%
Weighted average rate of compensation increase for: |  |  |
Benefit obligations | 4.60% | 4.60% | 4.60%
Net pension cost | 4.60% | 4.60% | 4.60%
Healthcare cost trend rates: |  |  |
Immediate trend | 5.80% | 5.90% | 6.20%
Ultimate trend | 4.50% | 4.50% | 4.50%
Other Benefit Plans | Discount rate |  |  |
Healthcare cost trend rates: |  |  |
1% Increase | $ (9.4) |  |
1% Decrease | $ 11.7 |  |
Impact on defined benefit obligation, increase in assumption | 1.00% |  |
Impact on defined benefit obligation, decrease in assumption | 1.00% |  |
Other Benefit Plans | Healthcare cost trend rate |  |  |
Healthcare cost trend rates: |  |  |
1% Increase | $ 8.6 |  |
1% Decrease | $ (6.7) |  |
Impact on defined benefit obligation, increase in assumption | 1.00% |  |
Impact on defined benefit obligation, decrease in assumption | 1.00% |  |

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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 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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