Pension and Other Benefits - Narrative (Details) - CAD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Contributions made by the Company	$ 4	$ 9

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

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Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration