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Shareholders' contributions - Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended 51 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2017
Sep. 24, 2013
Disclosure of terms and conditions of share-based payment arrangement [line items]          
Par value per share (in usd per share) $ 1.5        
Authorized percentage of outstanding stock         5.00%
Purchase of treasury shares $ 15,725 $ 38,367 $ 4,772    
Treasury shares 5,826,116        
Share Repurchase Program          
Disclosure of terms and conditions of share-based payment arrangement [line items]          
Shares repurchased under program       8,421,549  
Employee share options exercised (in shares)       2,598,423  
Purchase of treasury shares $ 15,725 $ 38,367 $ 4,772