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Share-based compensation - Share and performance-based compensation expense (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share and performance-based compensation expense [Abstract]      
Performance-based share expense $ 3,968 $ 2,402 $ 2,645
Service-based share expense 3,433 2,105 428
Total share-based compensation expense before income tax 7,401 4,507 3,073
Income tax 0 0 0
Total share-based compensation expense, net of income tax 7,401 $ 4,507 $ 3,073
Unrecognized compensation expense $ 3,560    
Weighted average period of expense recognition 2 years