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Shareholders' equity and share-based payments - Share cancellation (Details)
12 Months Ended
Dec. 12, 2018
shares
Dec. 15, 2016
subsidiary
shares
Dec. 31, 2018
shares
Dec. 31, 2016
shares
Subsidiaries from which cancelled treasury shares were previously bought back        
Shareholders' equity        
Number of indirectly controlled subsidiaries | subsidiary   4    
Cumulative percentage of interest held   100.00%    
Treasury shares        
Shareholders' equity        
Cancellation of treasury shares | shares 44,590,699 100,331,268 44,590,699 100,331,268