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Stockholders' equity - Reduction of Authorized Shares (Details) - shares
Jun. 30, 2018
Dec. 31, 2017
Conversion of Stock [Line Items]    
Authorized Shares of Common Stock 50,000,000 200,000,000
Original Number of Shares Authorized [Member]    
Conversion of Stock [Line Items]    
Authorized Shares of Common and Preferred Stock 240,000,000  
Current Number of Shares Authorized [Member]    
Conversion of Stock [Line Items]    
Authorized Shares of Common and Preferred Stock 55,000,000  
Authorized Shares of Preferred Stock 5,000,000  
Authorized Shares of Common Stock 50,000,000