Summary of significant accounting policies and recent accounting pronouncements - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended	6 Months Ended		12 Months Ended
	Feb. 03, 2015	Feb. 28, 2015	Jun. 30, 2016	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Subsidiary, Sale of Stock [Line Items]
Refund period from day of shipment (in days)				21 days
Advertising expense			$ 916,000	$ 1,426,000
Shipping and handling costs			$ 28,000	28,000		$ 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