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Commitments and contingencies (Details) - USD ($)
1 Months Ended 12 Months Ended
Jan. 31, 2019
Mar. 31, 2014
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2014
Operating Leased Assets [Line Items]            
Future minimum lease payments     $ 2,350      
Rent expense     351,000 $ 522,000 $ 337,000  
Percent of gross sales due to related parties   2.00%       2.00%
Royalty expense     17,000 25,000 20,000  
Royalties payable     $ 3,000 $ 3,000 $ 4,000  
Subsequent Event            
Operating Leased Assets [Line Items]            
Royalty agreement term 20 years          
Subsequent Event | Royalty agreement first ten years            
Operating Leased Assets [Line Items]            
Percent of gross sales due to related parties 2.00%          
Subsequent Event | Royalty agreement next ten years            
Operating Leased Assets [Line Items]            
Percent of gross sales due to related parties 1.00%