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Common stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 6 Months Ended
Jun. 30, 2016
Jan. 31, 2015
Jun. 30, 2016
Jun. 30, 2015
Equity [Abstract]        
Common stock issued (in shares) 8,222,500 2,645,000    
Common stock issue price (in dollars per share) $ 7 $ 50 $ 7  
Shares issued upon full exercise of options granted to underwriters (in shares) 1,072,500 345,000    
Net proceeds from offering $ 53,900 $ 124,100 $ 53,949 $ 124,063