Annual report pursuant to Section 13 and 15(d)

Summary of significant accounting policies - Additional Information (Details)

v3.19.1
Summary of significant accounting policies - Additional Information (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
segment
Dec. 31, 2017
USD ($)
shares
Mar. 31, 2019
USD ($)
Mar. 31, 2018
USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Joint venture, percent owned 51.00%      
Allowance for doubtful accounts $ 26,800 $ 22,400    
Impairment of long-lived assets $ 0      
Dilutive shares resulting from stock option exercises included in the calculation of EPS (shares) | shares   171,594    
Number of operating segments | segment 2      
Advertising expense $ 273,000 $ 278,000    
Research and development $ 1,298,000 $ 937,000    
Accumulated Deficit | Adjustments for New Accounting Pronouncement        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of new accounting principle       $ 165,317
Accumulated Other Comprehensive Loss | Adjustments for New Accounting Pronouncement        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cumulative effect of new accounting principle       $ 165,317
Forecast | Adjustments for New Accounting Pronouncement        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Right-of-use assets     $ 2,600,000  
Lease liabilities     $ 2,600,000