Pensions and Other Benefits - Amounts Recognized in Accumulated Other Comprehensive Losses (Details) - CAD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Amounts recognized in accumulated other comprehensive loss	$ (2,580)	$ (2,094)
Pension Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment gains	3,434	2,233
Deferred investment gains	41	611
Prior service cost	1	0
Deferred income tax	(964)	(797)
Amounts recognized in accumulated other comprehensive loss	2,512	2,047
Other Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment gains	91	61
Deferred investment gains	0	0
Prior service cost	1	2
Deferred income tax	(24)	(16)
Amounts recognized in accumulated other comprehensive loss	$ 68	$ 47