Pensions and Other Benefits - Additional Information (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Period of average market values for calculating net periodic benefit cost	5 years
Defined Benefit Plan, Accumulated Benefit Obligation	$ 12,201	$ 10,981
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation	419	395
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Plan Assets	186	$ 180
Effect of one percentage point increase on post-retirement benefit obligation	(5)
Effect of one percentage point decrease on post-retirement benefit obligation	$ 5
Maximum value of underlying assets represented by financial derivatives, excluding currency forwards	30.00%
Solvency funded position hedged against interest rate risk	45.00%	11.00%
Company's common shares in fund assets	119,758	86,084
Net cost of defined contribution plan	$ 11	$ 10	$ 9
Multi-employer plan contributions	$ 3	$ 3	$ 5
US Dollar
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	39.00%	33.00%
Percentage of plan exposed to foreign currency risk, excluding currency forwards	41.00%	43.00%
Euro
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	6.00%	4.00%
Other Currencies
Defined Benefit Plan Disclosure [Line Items]
Plans exposure to foreign currency risk percentage	14.00%	13.00%
Public equities
Defined Benefit Plan Disclosure [Line Items]
Company's securities in fund assets	$ 40	$ 21
Defined Benefit Plan, Debt Security
Defined Benefit Plan Disclosure [Line Items]
Company's securities in fund assets		$ 1
Pension Plan, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	176
Unamortized prior service cost expected to be recognized in next fiscal year	1
Other Post-retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	$ 3

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