Pensions and Other Benefits - Amounts Recognized in Accumulated Other Comprehensive Losses (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Amounts recognized in accumulated other comprehensive loss	$ 2,878	$ 2,580
Pension Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment gains	3,960	3,434
Deferred investment gains	(95)	41
Prior service cost	5	1
Deferred income tax	(1,070)	(964)
Amounts recognized in accumulated other comprehensive loss	2,800	2,512
Other Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment gains	104	91
Deferred investment gains	0	0
Prior service cost	1	1
Deferred income tax	(27)	(24)
Amounts recognized in accumulated other comprehensive loss	$ 78	$ 68