Pensions and Other Benefits - Narrative (Details) - Pension Plans, Defined Benefit [Member] - CAD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Contributions made by the company	$ 13	$ 11	$ 25	$ 35
Refund of plan surplus			$ (10)

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

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