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Stockholders' equity (Details) - $ / shares
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Preferred stock    
Preferred stock, shares authorized (in shares) 15,000,000 15,000,000
Preferred stock, par value (in dollars per share) $ 0.001 $ 0.001
Common stock    
Common stock, par value (in dollars per share) $ 0.001 $ 0.001
Common stock, shares authorized (in shares) 200,000,000 100,000,000
Common stock, voting rights 1  
Senior Managment [Member]    
Treasury Stock [Abstract]    
Stock repurchased as part of stock swap (in shares)