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Employee benefits - Defined contribution plans (Details)
$ in Millions, $ in Millions
12 Months Ended
Dec. 31, 2017
CAD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Employee benefits      
Costs related to defined contribution plans   $ 16.7 $ 14.5
Defined Contribution Plans      
Employee benefits      
Costs related to defined contribution plans   15.2 14.1
Executive Vice President and CFO      
Employee benefits      
Costs related to defined contribution plans   1.5 $ 0.4
Estimated contributions expected to be paid $ 5.3 4.2  
Employer contribution, quarterly amount $ 0.5 $ 0.4  
Term of employer contribution towards defined contribution plan 33 months 33 months