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Summary of significant accounting policies and recent accounting pronouncements - Narrative (Details) - USD ($)
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Feb. 03, 2015
Feb. 28, 2015
Sep. 30, 2016
Jun. 30, 2016
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Subsidiary, Sale of Stock [Line Items]              
Refund period from day of shipment (in days)         21 days    
Advertising expense     $ 1,154,000   $ 2,580,000    
Shipping and handling costs     $ 98,000 $ 98,000 126,000   $ 0
Proceeds from IPO, net of offering costs $ 79,900,000 $ 79,900,000     $ 0 $ 80,435,430  
Common stock, number of shares issued from conversion of preferred stock (in shares) 6,971,108            
Common stock, shares authorized (in shares)     100,000,000   100,000,000   100,000,000
IPO issuance cost $ 1,848,737            
IPO              
Subsidiary, Sale of Stock [Line Items]              
Common stock, shares issued 5,491,191 5,491,191          
Common stock, price per share (in usd per share) $ 16.00 $ 16.00          
Over-Allotment Option              
Subsidiary, Sale of Stock [Line Items]              
Common stock, shares issued 91,191 91,191