Pensions and Other Benefits - Information About DB Pension Plans and Other Benefits (Details) - CAD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in fund assets:
Fair value of fund assets at January 1	$ 12,349
Fair value of fund assets at December 31	13,319	$ 12,349
Pension Plan, Defined Benefit
Change in projected benefit obligation:
Benefit obligation at January 1	11,372	11,679
Current service cost	107	120	$ 103
Interest cost	450	438	451
Employee contributions	41	47
Benefits paid	(646)	(640)
Foreign currency changes	(10)	20
Actuarial loss (gain)	1,296	(292)
Projected benefit obligation at December 31	12,610	11,372	11,679
Change in fund assets:
Fair value of fund assets at January 1	12,349	12,808
Actual return on fund assets	1,528	82
Employer contributions	53	36
Employee contributions	41	47
Benefits paid	(646)	(640)
Foreign currency changes	(6)	16
Fair value of fund assets at December 31	13,319	12,349	12,808
Funded status - plan surplus (deficit)	709	977
Other Post-retirement Benefit Plans
Change in projected benefit obligation:
Benefit obligation at January 1	501	518
Current service cost	11	12	12
Interest cost	20	19	20
Employee contributions	0	1
Benefits paid	(34)	(33)
Foreign currency changes	0	2
Actuarial loss (gain)	43	(18)
Projected benefit obligation at December 31	541	501	518
Change in fund assets:
Fair value of fund assets at January 1	4	4
Actual return on fund assets	1	0
Employer contributions	34	32
Employee contributions	0	1
Benefits paid	(34)	(33)
Foreign currency changes	0	0
Fair value of fund assets at December 31	5	4	$ 4
Funded status - plan surplus (deficit)	$ (536)	$ (497)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Amount of funded (unfunded) status of defined benefit plan, measured as difference between fair value of plan assets and benefit obligation. Includes, but is not limited to, overfunded (underfunded) status.

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