PENSION AND OTHER POSTRETIREMENT BENEFITS - AMOUNT RECOGNIZED IN EXCESS OF ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Canada
Pension and Other Postretirement Benefit Disclosures
Projected benefit obligations	$ 4,434	$ 1,481
Accumulated benefit obligations	4,094	1,361
Fair value of plan assets	3,621	1,087
Canada \| OPEB
Pension and Other Postretirement Benefit Disclosures
Accumulated benefit obligations	321	293
Fair value of plan assets	0	0
United States
Pension and Other Postretirement Benefit Disclosures
Projected benefit obligations	1,243	103
Accumulated benefit obligations	1,207	98
Fair value of plan assets	1,062	0
United States \| OPEB
Pension and Other Postretirement Benefit Disclosures
Accumulated benefit obligations	191	288
Fair value of plan assets	$ 106	$ 188

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

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