PENSIONS AND OTHER POST RETIREMENT BENEFITS - Actuarial assumptions (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Disclosure of defined benefit plans [line items]
Actual experience period	5 years
Increase in actuarial assumption (as a percent)	1.00%
Decrease in actuarial assumption (as a percent)	1.00%
Defined benefit pension plans
Disclosure of defined benefit plans [line items]
Discount rate	2.90%	2.50%
Rate of compensation increase	3.00%	3.00%
Defined benefit pension plans \| Discount rate
Disclosure of defined benefit plans [line items]
Increase (decrease) in interest and service cost due to increase in actuarial assumption	$ (24)
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	(1,155)
Increase (decrease) in net obligation due to decrease in actuarial assumption	$ 1,515
Defined benefit other post retirement benefits
Disclosure of defined benefit plans [line items]
Discount rate	2.90%	2.50%
Rate of compensation increase	3.00%	3.00%
Assumption of percentage of increase of healthcare costs	5.00%
Increase (decrease) in net obligation due to increase in actuarial assumption	$ (80)
Increase (decrease) in net obligation due to decrease in actuarial assumption	99
Defined benefit other post retirement benefits \| Actuarial assumption of medical cost trend rates
Disclosure of defined benefit plans [line items]
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	36
Increase (decrease) in net obligation due to decrease in actuarial assumption	$ (31)

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
-Name IAS
-Number 19
-IssueDate 2021-01-01
-Paragraph 144
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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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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

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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
-Name IAS
-Number 19
-IssueDate 2021-01-01
-Paragraph 145
-Subparagraph a
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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]]

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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]]

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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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