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Bank and other loans - Additional Information (Details) - USD ($)
 12 Months Ended
Jul. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 29, 2016
Jun. 28, 2016
Disclosure of detailed information about borrowings [line items]            
Finance costs   $ 9,200,000 $ 9,300,000 $ 10,800,000    
Bank and other loans   108,800,000 121,000,000 $ 131,600,000    
Senior Facilities Agreement [Member] | Gross carrying amount            
Disclosure of detailed information about borrowings [line items]            
Notional amount $ 150,000,000          
Senior Facilities Agreement, Accordion Feature [Member] | Gross carrying amount            
Disclosure of detailed information about borrowings [line items]            
Notional amount 50,000,000          
Finance costs $ 1,200,000          
Loan Notes due 2018            
Disclosure of detailed information about borrowings [line items]            
Bank and other loans   15,000,000 14,900,000      
Loan Notes due 2018 | Gross carrying amount            
Disclosure of detailed information about borrowings [line items]            
Notional amount         $ 65,000,000  
Bank and other loans   15,000,000 15,000,000   $ 50,000,000  
Loan Notes due 2023            
Disclosure of detailed information about borrowings [line items]            
Bank and other loans   24,700,000 24,700,000      
Borrowings, interest rate         4.88% 6.19%
Loan Notes due 2023 | Gross carrying amount            
Disclosure of detailed information about borrowings [line items]            
Bank and other loans   25,000,000 25,000,000      
Loan Notes due 2026            
Disclosure of detailed information about borrowings [line items]            
Bank and other loans   24,700,000 24,700,000      
Borrowings, interest rate         4.94%  
Loan Notes due 2026 | Gross carrying amount            
Disclosure of detailed information about borrowings [line items]            
Bank and other loans   25,000,000 25,000,000      
Loan Notes due 2021            
Disclosure of detailed information about borrowings [line items]            
Bank and other loans   $ 24,900,000 24,900,000      
Borrowings, interest rate   3.67%        
Borrowings, term   7 years        
Loan Notes due 2021 | Gross carrying amount            
Disclosure of detailed information about borrowings [line items]            
Bank and other loans   $ 25,000,000 25,000,000      
Revolving credit facility            
Disclosure of detailed information about borrowings [line items]            
Bank and other loans   19,500,000 31,800,000      
Revolving credit facility | Gross carrying amount            
Disclosure of detailed information about borrowings [line items]            
Bank and other loans   $ 21,300,000 $ 32,800,000