Derivatives (Narrative) (Details) gal in Millions, CAD in Millions	6 Months Ended
Jun. 30, 2017 CAD gal
Diesel swap
Derivative [Line Items]
Fixed price swap contract (in gallons) \| gal	4.4
Foreign currency forward contracts
Derivative [Line Items]
Derivative purchases of underlying currency (in Canadian Dollars) \| CAD	CAD 402

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