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Capital structure (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended
Oct. 16, 2014
Feb. 28, 2014
Dec. 31, 2016
Dec. 31, 2015
May 31, 2013
Capital structure          
Common stock, authorized shares (in shares)     125,000,000 125,000,000  
Common stock          
Capital structure          
Common stock, authorized shares (in shares)         125,000,000
Shares issued and sold (in shares) 3,450,000 5,163,265      
Public offering price (in dollars per share) $ 36.25 $ 24.50      
Common stock shares issuable pursuant to the exercise by the underwriters of an over-allotment option (in shares) 450,000 673,469      
Net proceeds from public offerings $ 117.6 $ 118.4