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Stockholders' equity, Preferred stock and Common stock (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Preferred stock [Abstract]        
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 [Abstract]        
Common stock, par value (in dollars per share) $ 0.001 $ 0.001 $ 0.001 $ 0.001
Common stock, shares authorized (in shares) 200,000,000 200,000,000    
Common stock, voting rights 1