Pensions and Other Benefits - Additional Information (Details) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Period of average market values for calculating net periodic benefit cost	5 years
DB Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation	$ 443	$ 419
DB Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets	$ 187	$ 186
Maximum value of underlying assets represented by financial derivatives, excluding currency forwards	30.00%
Solvency funded position hedged against interest rate risk	47.00%	45.00%
Company's common shares in fund assets	109,008	119,758
Net cost of defined contribution plan	$ 12	$ 11	$ 10
Multi-employer benefit plan, Employer contributions	$ 3	$ 3	$ 3
US Dollar
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	33.00%	39.00%
Percentage of plan exposed to foreign currency risk, excluding currency forwards	40.00%	41.00%
Euro
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	6.00%	6.00%
Other Currencies
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	14.00%	14.00%
Public equities
Defined Benefit Plan Disclosure [Line Items]
Company's securities in fund assets	$ 48	$ 40
Pension Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
DB pension plans' accumulated benefit obligation	$ 13,528	$ 12,201