Annual report pursuant to Section 13 and 15(d)

Summary of significant accounting policies - Concentration of Credit Risk (Details)

v3.8.0.1
Summary of significant accounting policies - Concentration of Credit Risk (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
customer
Dec. 31, 2016
USD ($)
customer
Concentration Risk [Line Items]    
Cash, FDIC Insured Amount | $ $ 250,000  
Cash, Uninsured Amount | $ $ 1,172,911 $ 3,471,765
Customer concentration risk | Revenues    
Concentration Risk [Line Items]    
Number of customer representing more than 10% of revenues or trade accounts receivable 1 0
Number of customers 470  
Customer concentration risk | Trade accounts receivable    
Concentration Risk [Line Items]    
Number of customer representing more than 10% of revenues or trade accounts receivable 0  
Concentration risk, percentage 10.00% 15.00%