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Shareholders' contributions - Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended 51 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
Sep. 24, 2013
Disclosure of terms and conditions of share-based payment arrangement [line items]          
Stock Repurchase Program, Authorized Percentage of Outstanding Stock         5.00%
Par value per share (in usd per share) $ 1.5     $ 1.5  
Purchase of treasury shares $ 38,367 $ 4,772 $ 319    
Treasury shares 4,643,396     4,643,396  
Share Repurchase Program          
Disclosure of terms and conditions of share-based payment arrangement [line items]          
Shares repurchased under program       6,742,183  
Employee share options exercised (in shares)       2,101,777  
Purchase of treasury shares $ 38,367 $ 4,772 $ 320