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Retirement plans - Additional information (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
year
Dec. 31, 2016
USD ($)
Disclosure of net defined benefit liability (asset) [line items]    
Retirement obligation (asset) $ 18,402 $ 16,541
Weighted average duration of defined benefit obligation | year 10  
Unfunded | Foreign | Chile    
Disclosure of net defined benefit liability (asset) [line items]    
Retirement obligation (asset) $ 25,100 22,400
Estimate of contributions expected to be paid to plan for 2018 5,100  
Funded | Domestic    
Disclosure of net defined benefit liability (asset) [line items]    
Retirement obligation (asset) (6,600) (5,700)
Estimate of contributions expected to be paid to plan for 2018 600  
Funded | Foreign | Europe    
Disclosure of net defined benefit liability (asset) [line items]    
Retirement obligation (asset) $ (100) $ (200)