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Payroll costs, share based payments and management incentive schemes - Information About the Number of Persons Employed by the Company (Details) - employee
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Employee Information [Line Items]      
Average number of employees 4,756 4,425 3,857
Production      
Employee Information [Line Items]      
Average number of employees 3,308 2,915 2,285
Administration, distribution & sales      
Employee Information [Line Items]      
Average number of employees 1,448 1,510 1,572