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PENSION AND OTHER POSTRETIREMENT BENEFITS - ASSUMED HEALTH CARE COST TREND RATES (Details) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Canada |  |
MEDICAL COST TRENDS |  |
Health care cost trend rate assumed for next year | 5.60% | 5.50%
Rate to which the cost trend is assumed to decline (the ultimate trend rate) | 4.40% | 4.40%
Year that the rate reaches the ultimate trend rate | 2034 | 2034
Effect of 1% change in assumed medical care trend rate |  |
Increase in service and interest cost due to 1% increase in the assumed medical care trend rate | $ 1 |
Decrease in service and interest cost due to 1% decrease in the assumed medical care trend rate | (1) |
Increase in benefit obligation due to 1% increase in the assumed medical care trend rate | 20 |
Decrease in benefit obligation due to 1% decrease in the assumed medical care trend rate | $ (16) |
United States |  |
MEDICAL COST TRENDS |  |
Health care cost trend rate assumed for next year | 7.40% | 7.40%
Rate to which the cost trend is assumed to decline (the ultimate trend rate) | 4.50% | 4.50%
Year that the rate reaches the ultimate trend rate | 2037 | 2037
Effect of 1% change in assumed medical care trend rate |  |
Increase in service and interest cost due to 1% increase in the assumed medical care trend rate | $ 1 |
Decrease in service and interest cost due to 1% decrease in the assumed medical care trend rate | (1) |
Increase in benefit obligation due to 1% increase in the assumed medical care trend rate | 18 |
Decrease in benefit obligation due to 1% decrease in the assumed medical care trend rate | $ (17) |

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- Definition Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (m) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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- Definition Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 + Details
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