Convertible preferred stock - Narrative (Details) - USD ($)	3 Months Ended	4 Months Ended	10 Months Ended	12 Months Ended
	May. 31, 2014	Oct. 31, 2014	Dec. 31, 2014	Dec. 31, 2015
Class of Stock [Line Items]
Preferred stock, authorized (shares)			0	10,000,000
Preferred stock, par value (in usd per share)				$ 0.0001
Preferred stock, carrying value			$ 0	$ 0
Series A convertible preferred stock
Class of Stock [Line Items]
Preferred stock, authorized (shares)			16,000,000	0
Preferred stock, par value (in usd per share)			$ 0.0001
Issuance of stock/IPO proceeds, net of issuance costs (in shares)	15,775,221
Convertible preferred stock, automatic conversion, minimum share price (in usd per share)	$ 1.00
Net proceeds from issuance of convertible preferred stock	$ 15,637,032		$ 15,581,811	$ 0
Preferred stock, carrying value			$ 15,637,032	$ 0
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, authorized (shares)			14,500,000	0
Preferred stock, par value (in usd per share)			$ 0.0001
Issuance of stock/IPO proceeds, net of issuance costs (in shares)		14,078,647
Convertible preferred stock, automatic conversion, minimum share price (in usd per share)		$ 1.81
Net proceeds from issuance of convertible preferred stock		$ 25,394,135	$ 25,394,135	$ 0
Preferred stock, carrying value			$ 25,394,135	$ 0