Pensions and Other Benefits - Amounts Recognized in Company's Consolidated Balance Sheets (Details) - CAD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Pension asset	$ 1,003	$ 1,243
Pension and other benefit liabilities	(785)	(718)
Pension Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Pension asset	1,003	1,243
Accounts payable and accrued liabilities	(11)	(11)
Pension and other benefit liabilities	(283)	(255)
Total amount recognized	709	977
Other Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Pension asset	0	0
Accounts payable and accrued liabilities	(34)	(34)
Pension and other benefit liabilities	(502)	(463)
Total amount recognized	$ (536)	$ (497)