Annual report pursuant to Section 13 and 15(d)

Stockholders' equity - Underwritten public offering and Registered direct offering (Details)

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Stockholders' equity - Underwritten public offering and Registered direct offering (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Apr. 10, 2017
Mar. 27, 2017
May 11, 2015
Dec. 31, 2017
Jun. 30, 2015
Stockholders' equity          
Proceeds from issuance of common stock       $ 103,167  
Common stock          
Stockholders' equity          
Issuance of common stock (in shares)       136,201,338  
Stock offering expenses     $ 13,387 $ 4,774 $ 13,387
Underwritten public offering          
Stockholders' equity          
Issuance of shares represented by American Depositary Shares (in ADSs)   15,700,223      
Share price per ADS (in dollars per share)   $ 4.20      
Underwritten public offering | Common stock          
Stockholders' equity          
Issuance of common stock (in shares)   94,201,338      
Proceeds from issuance of common stock   $ 61,397      
Stock offering expenses   $ 4,544      
Matrix Capital Management Company, LP          
Stockholders' equity          
Issuance of shares represented by American Depositary Shares (in ADSs) 7,000,000        
Share price per ADS (in dollars per share) $ 6.00        
Matrix Capital Management Company, LP | Common stock          
Stockholders' equity          
Issuance of common stock (in shares) 42,000,000        
Proceeds from issuance of common stock $ 41,770        
Stock offering expenses $ 230