Summary of significant accounting policies - Narrative (Details)		1 Months Ended		3 Months Ended	9 Months Ended	10 Months Ended	12 Months Ended
	Feb. 03, 2015 USD ($) $ / shares shares	Feb. 28, 2015 USD ($) $ / shares shares	Jan. 31, 2015	Dec. 31, 2016 shares	Sep. 30, 2016	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($) segment shares	Dec. 31, 2015 USD ($) shares
Entity Location [Line Items]
Number of operating segments | segment							2
Revenue recognition, refund period				30 days	21 days
Shipping and handling costs							$ 170,000	$ 0
Advertising expenses							2,936,000	0
IPO proceeds, net of issuance costs	$ 79,900,000	$ 79,900,000				$ 0	$ 0	$ 80,435,430
Common stock, number of shares issued from conversion of preferred stock (in shares) | shares	6,971,108
Common stock, shares authorized (shares) | shares				100,000,000			100,000,000	100,000,000
IPO issuance costs	$ 1,848,737
IPO underwriting discounts and commissions	$ 6,150,134
Reverse stock split conversion ratio			0.2335
IPO
Entity Location [Line Items]
Common stock, shares issued | shares	5,491,191	5,491,191
Common stock, price per share (in usd per share) | $ / shares	$ 16.00	$ 16.00
Over-Allotment Option
Entity Location [Line Items]
Common stock, shares issued | shares	91,191	91,191
United States
Entity Location [Line Items]
Number of geographic segments | segment							1