Pensions and Other Benefits - Changes in Fund Assets (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of fund assets at January 1	$ 13,319
Fair value of fund assets at December 31	14,365	$ 13,319
Pension Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Fair value of fund assets at January 1	13,319	12,349
Actual return on fund assets	1,634	1,528
Employer contributions	27	53
Employee contributions	42	41
Benefits paid	(653)	(646)
Foreign currency changes	(4)	(6)
Fair value of fund assets at December 31	14,365	13,319
Funded status – plan surplus (deficit)	566	709
Other Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of fund assets at January 1	5	4
Actual return on fund assets	0	1
Employer contributions	34	34
Employee contributions	0	0
Benefits paid	(34)	(34)
Foreign currency changes	0	0
Fair value of fund assets at December 31	5	5
Funded status – plan surplus (deficit)	$ (548)	$ (536)