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Other long-term liabilities and deferred credits - Changes in Environmental Remediation Obligation (Details) - CAD
CAD in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Accrual for Environmental Loss Contingencies [Roll Forward]    
Opening Balance CAD 63,378 CAD 71,529
Remediation activities (2,026) (1,389)
Accretion 1,447 2,464
Changes in cash flow estimates 2,135 2,088
Revision in assumptions 7,686 (9,101)
Foreign exchange rate adjustment (4,473) (2,213)
Closing Balance CAD 68,147 CAD 63,378