POST-EMPLOYMENT BENEFITS - Summary of Pension Fund Assets (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Changes in net defined benefit liability (asset) [abstract]
Plan assets, beginning of year	$ (73)
Remeasurements, recognized in other comprehensive income and equity	240	$ 355
Contributions by employees	34	36
Contributions by employer	150	179
Plan assets, end of year	(92)	(73)
Domestic defined benefit plans
Changes in net defined benefit liability (asset) [abstract]
Plan assets, beginning of year	(451)
Interest income	(10)	(8)
Remeasurements, recognized in other comprehensive income and equity	163	277
Administrative expenses paid from plan assets	(4)	(4)
Plan assets, end of year	(574)	(451)
Domestic defined benefit plans \| Plan assets
Changes in net defined benefit liability (asset) [abstract]
Plan assets, beginning of year	2,449	1,965
Interest income	81	81
Remeasurements, recognized in other comprehensive income and equity	163	277
Contributions by employees	34	36
Contributions by employer	150	179
Benefits paid	(82)	(86)
Administrative expenses paid from plan assets	(4)	(3)
Plan assets, end of year	$ 2,791	$ 2,449

No definition available.

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- Definition

The decrease (increase) in net defined benefit liability (asset) resulting from contributions to a defined benefit plan by the employer. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-Number 19
-IssueDate 2020-01-01
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from contributions to a defined benefit plan by plan participants. [Refer: Net defined benefit liability (asset); Defined benefit plans [member]]

+ References

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- Definition

The increase (decrease) in the net defined benefit liability (asset) resulting from the passage of time. [Refer: Interest expense; Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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-IssueDate 2020-01-01
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- Definition

The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from payments from the plan. [Refer: Net defined benefit liability (asset)]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
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-IssueDate 2020-01-01
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- Definition

The decrease (increase) in the net defined benefit liability (asset) resulting from the return on plan assets, excluding amounts included in interest income or expense. The return on plans assets is interest, dividends and other revenue derived from the plan assets, together with realised and unrealised gains or losses on the plan assets, less any costs of managing plan assets and less any tax payable by the plan itself, other than tax included in the actuarial assumptions used to measure the present value of the defined benefit obligation. [Refer: Plan assets [member]; Net defined benefit liability (asset); Actuarial assumptions [member]; Interest expense (income), net defined benefit liability (asset)]

+ References

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- Definition

Administrative Expenses, Net Defined Benefit Liability (Asset)

+ References

No definition available.

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