Pensions and Other Postretirement Benefits (Contributions to Multi-employer Plan) (Details) - Railroad Employees National Early Retirement Major Medical Benefit Plan [Member] CAD in Millions	12 Months Ended
Dec. 31, 2017 CAD CAD / PerMonthPerEmployee	Dec. 31, 2016 CAD CAD / PerMonthPerEmployee	Dec. 31, 2015 CAD
Contributions to multiemployer plan [Line Items]
Contributions to multi-employer plan \| CAD	CAD 15	CAD 12	CAD 10
Multiemployer plan employer contribution rate per month \| CAD / PerMonthPerEmployee	216.54	178.45
Number of retirees covered	462	416

Amount of contributions made to multiemployer plans by the employer. A multiemployer plan is a pension or postretirement benefit plan to which two or more unrelated employers contribute where assets contributed by one participating employer may be used to provide benefits to employees of other participating employers.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450702-114947

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450691-114947

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 80
-Section 50
-Paragraph 5
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=65877416&loc=SL14450657-114947

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