Pensions and Other Benefits - Narrative (Details) - Pension Plans, Defined Benefit [Member] - CAD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Contributions made by the company	$ 11	$ 1
Refund of plan surplus		$ (10)

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