PENSIONS AND OTHER POST RETIREMENT BENEFITS - Actuarial assumptions (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
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	3.10%	3.80%
Rate of compensation increase	3.00%	3.00%
Defined benefits pension plans \| Discount rate
Pension and other PostRetirement Benefits,
Increase (decrease) in interest and service cost due to increase in actuarial assumption	$ (25)
Increase (decrease) in service and interest cost due to decrease in actuarial assumption	31
Increase (decrease) in net obligation due to increase in actuarial assumption	(988)
Increase (decrease) in net obligation due to decrease in actuarial assumption	$ 1,271
Defined benefits other post retirement benefits
Pension and other PostRetirement Benefits,
Discount rate	3.10%	3.90%
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	$ (74)
Increase (decrease) in net obligation due to decrease in actuarial assumption	91
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	$ (26)

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]]

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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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]]

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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]]

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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
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-Number 19
-IssueDate 2019-01-01
-Paragraph 145
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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]]

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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]]

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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]]

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Actual experience period.

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Medical cost trend rate used as significant actuarial assumption to determine the present value of a defined benefit obligation after year five.

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Increase (decrease) in service and interest cost due to decrease in actuarial assumption.

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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.

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percentage of decrease in healthcare cost in year six

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