Pensions and other benefits - Amounts Recognized in Accumulated Other Comprehensive Losses (Details) - CAD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Amounts recognized in accumulated other comprehensive loss	$ 1,915	$ 2,878
Pensions
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment gains	3,298	3,960
Deferred investment gains	(672)	(95)
Prior service cost	5	5
Deferred income tax	(759)	(1,070)
Amounts recognized in accumulated other comprehensive loss	1,872	2,800
Other benefits
Defined Benefit Plan Disclosure [Line Items]
Other than deferred investment gains	57	104
Deferred investment gains	0	0
Prior service cost	1	1
Deferred income tax	(15)	(27)
Amounts recognized in accumulated other comprehensive loss	$ 43	$ 78