Pensions and Other Benefits - Changes in Projected Benefit Obligation (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in projected benefit obligation:
Benefit obligation, Discount rate	2.58%	3.25%	4.01%
Pension Plan, Defined Benefit
Change in projected benefit obligation:
Benefit obligation at January 1	$ 12,610	$ 11,372
Current service cost	140	107	$ 120
Interest cost	406	450	438
Employee contributions	42	41
Benefits paid	(653)	(646)
Foreign currency changes	(5)	(10)
Plan amendments and other	3	0
Actuarial loss	1,256	1,296
Projected benefit obligation at December 31	$ 13,799	$ 12,610	$ 11,372
Benefit obligation, Discount rate	2.58%	3.25%	4.01%
Other Post-retirement Benefit Plans
Change in projected benefit obligation:
Benefit obligation at January 1	$ 541	$ 501
Current service cost	12	11	$ 12
Interest cost	17	20	19
Employee contributions	0	0
Benefits paid	(34)	(34)
Foreign currency changes	0	0
Plan amendments and other	0	0
Actuarial loss	17	43
Projected benefit obligation at December 31	$ 553	$ 541	$ 501