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Summary of significant accounting policies and recent accounting pronouncements - Narrative (Details) - USD ($)
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Refund period from day of shipment (in days) 30 days    
Advertising expense $ 665,000 $ 0  
Shipping and handling costs 34,000 $ 0  
Accounting Standards Update 2016-09      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Deferred tax assets 50,000    
Retained Earnings | Accounting Standards Update 2016-09      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Cumulative effect adjustment $ 50,000   $ 2,000