Pensions and Other Benefits - Amounts Recognized in Company's Consolidated Balance Sheets (Details) - CAD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Pension asset	$ 894	$ 1,003
Pension and other benefit liabilities	(832)	(785)
Pension Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Pension asset	894	1,003
Accounts payable and accrued liabilities	(11)	(11)
Pension and other benefit liabilities	(317)	(283)
Total amount recognized	566	709
Other Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Pension asset	0	0
Accounts payable and accrued liabilities	(33)	(34)
Pension and other benefit liabilities	(515)	(502)
Total amount recognized	$ (548)	$ (536)