Net Interest Expense - Narrative (Details) - CAD CAD in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Income (Expense), Net [Abstract]
Interest on capital leases included in interest expense	CAD 11	CAD 11	CAD 11

The financing charge for leasing arrangements meeting the criteria for capitalization.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84164758&loc=d3e45031-112735

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