EMPLOYEE POST-RETIREMENT BENEFITS - Obligations, Fair Value and Weighted Average Assets (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Benefit obligation and fair value of plan assets for plans that are not fully funded
Plan assets at fair value	$ 4,099	$ 3,697
Pension plans' weighted average asset allocations and target allocations by asset category
Percentage of Plan Assets	100.00%	100.00%
Pension Benefit Plans
Benefit obligation and fair value of plan assets for plans that are not fully funded
Projected benefit obligation	$ (4,058)	$ (3,653)	$ (3,646)
Plan assets at fair value	3,693	3,321	3,451
Funded Status – Plan Deficit	(365)	(332)
Funded status based on accumulated benefit obligation
Accumulated benefit obligation	(3,719)	(3,347)
Plan assets at fair value	3,693	3,321
Funded Status	$ (26)	$ (26)
Pension plans' weighted average asset allocations and target allocations by asset category
Percentage of Plan Assets	100.00%	100.00%
Pension Benefit Plans \| Debt securities
Pension plans' weighted average asset allocations and target allocations by asset category
Percentage of Plan Assets	32.00%	33.00%
Company debt or common shares included in plan assets, amount	$ 9	$ 8
Company debt or common shares included in plan assets, percentage	0.20%	0.30%
Pension Benefit Plans \| Debt securities \| Minimum
Pension plans' weighted average asset allocations and target allocations by asset category
Target Allocations	25.00%
Pension Benefit Plans \| Debt securities \| Maximum
Pension plans' weighted average asset allocations and target allocations by asset category
Target Allocations	45.00%
Pension Benefit Plans \| Equity securities
Pension plans' weighted average asset allocations and target allocations by asset category
Percentage of Plan Assets	58.00%	56.00%
Company debt or common shares included in plan assets, amount	$ 15	$ 7
Company debt or common shares included in plan assets, percentage	0.40%	0.20%
Pension Benefit Plans \| Equity securities \| Minimum
Pension plans' weighted average asset allocations and target allocations by asset category
Target Allocations	40.00%
Pension Benefit Plans \| Equity securities \| Maximum
Pension plans' weighted average asset allocations and target allocations by asset category
Target Allocations	70.00%
Pension Benefit Plans \| Alternatives
Pension plans' weighted average asset allocations and target allocations by asset category
Percentage of Plan Assets	10.00%	11.00%
Pension Benefit Plans \| Alternatives \| Minimum
Pension plans' weighted average asset allocations and target allocations by asset category
Target Allocations	5.00%
Pension Benefit Plans \| Alternatives \| Maximum
Pension plans' weighted average asset allocations and target allocations by asset category
Target Allocations	15.00%
Pension Benefit Plans \| Not fully funded
Benefit obligation and fair value of plan assets for plans that are not fully funded
Projected benefit obligation	$ (4,058)	$ (3,653)
Plan assets at fair value	3,693	3,321
Funded Status – Plan Deficit	(365)	(332)
Funded status based on accumulated benefit obligation
Accumulated benefit obligation	(2,397)	(3,347)
Plan assets at fair value	2,351	3,321
Funded Status	(46)	(26)
Other Post-Retirement Benefit Plans
Benefit obligation and fair value of plan assets for plans that are not fully funded
Projected benefit obligation	(427)	(430)	(375)
Plan assets at fair value	406	376	$ 365
Funded Status – Plan Deficit	(21)	(54)
Other Post-Retirement Benefit Plans \| Not fully funded
Benefit obligation and fair value of plan assets for plans that are not fully funded
Projected benefit obligation	(182)	(246)
Plan assets at fair value	0	0
Funded Status – Plan Deficit	$ (182)	$ (246)

The difference between the accumulated benefit obligation and fair value of plan assets.

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- Definition

The percentage of employer securities or securities of parties related to the employer that are included in plan assets.

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- Definition

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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- Definition

Amount of employer security in which defined benefit plan asset is invested. Includes, but is not limited to, security issued or managed by related party of employer.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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Amount of plan asset for defined benefit plan with accumulated benefit obligation in excess of plan assets.

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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