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PENSION AND OTHER POSTRETIREMENT BENEFITS - AMOUNT RECOGNIZED IN ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - CAD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Canada | Pension |  |
Amount included in AOCI |  |
Net actuarial loss (gain) | $ 542 | $ 445
Prior service credit | 0 | 0
Total amount recognized in AOCI | 542 | 445
Canada | OPEB |  |
Amount included in AOCI |  |
Net actuarial loss (gain) | 15 | (7)
Prior service credit | (1) | (1)
Total amount recognized in AOCI | 14 | (8)
United States | Pension |  |
Amount included in AOCI |  |
Net actuarial loss (gain) | 233 | 134
Prior service credit | (1) | (2)
Total amount recognized in AOCI | 232 | 132
United States | OPEB |  |
Amount included in AOCI |  |
Net actuarial loss (gain) | (7) | (23)
Prior service credit | (44) | (13)
Total amount recognized in AOCI | $ (51) | $ (36)

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