Commitments and Contingencies (Narrative) (Details) - Sep. 30, 2017 CAD in Millions, $ in Millions	CAD	USD ($)
Commitment And Contingencies [Line Items]
Purchase commitments		$ 356.0
Outstanding letters of credit		7.8
Credit Letter Agreement [Member]
Commitment And Contingencies [Line Items]
Outstanding letters of credit	CAD 3.5	2.8
Credit letter agreement limit	CAD 5.0	$ 4.0

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant