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Amounts Reclassified Out of Accumulated Other Comprehensive Income (Loss) (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Amortization of postretirement benefits liability adjustment included in net periodic benefit cost [1] $ (148) $ (185) $ (194)
[1] This accumulated other comprehensive income component is included in the computation of net periodic benefit cost (note 5).