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Earnings per share - Additional information (Details)
shares in Thousands
12 Months Ended
Dec. 31, 2017
category
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Earnings per share [abstract]      
Number of categories of dilutive potential shares | category 2    
Antidilutive securities | shares 851 1,658 1,701