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Shareholders' equity and share-based payments - Share cancellation (Details)
12 Months Ended
Dec. 15, 2016
subsidiary
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 100,331,268 100,331,268