Convertible preferred stock - Narrative (Details) (USD $)	10 Months Ended	12 Months Ended	3 Months Ended	4 Months Ended
	Dec. 31, 2014 director	Dec. 31, 2014	May 31, 2014	Oct. 31, 2014
Class of Stock [Line Items]
Convertible preferred stock, automatic conversion, minimum share price (in usd per share)		$ 8.57
Preferred stock, voting rights, number of directors entitled to elect	2
Preferred stock, minimum gross proceeds that would trigger an automatic conversion		$ 40,000,000
Series A convertible preferred stock
Class of Stock [Line Items]
Preferred stock, authorized (shares)	16,000,000	16,000,000
Preferred stock, par value (in usd per share)	0.0001	$ 0.0001
Issuance of convertible preferred stock, net of issuance costs (in shares)			15,775,221
Convertible preferred stock, automatic conversion, minimum share price (in usd per share)		$ 1.00	$ 1.00
Net proceeds from issuance of convertible preferred stock	15,581,811		15,637,032
Preferred stock, carrying value	15,637,032	15,637,032
Percent of preferred stockholders	75.00%	75.00%
Preferred stock, liquidation preference per share (in usd per share)	1.00	$ 1.00
Preferred stock, conversion price (in usd per share)	4.28	$ 4.28
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, authorized (shares)	14,500,000	14,500,000
Preferred stock, par value (in usd per share)	0.0001	$ 0.0001
Issuance of convertible preferred stock, net of issuance costs (in shares)				14,078,647
Convertible preferred stock, automatic conversion, minimum share price (in usd per share)		$ 1.81		$ 1.81
Net proceeds from issuance of convertible preferred stock	25,394,135			25,394,135
Preferred stock, carrying value	25,394,135	$ 25,394,135
Percent of preferred stockholders	60.00%	60.00%
Preferred stock, liquidation preference per share (in usd per share)	1.81	$ 1.81
Preferred stock, conversion price (in usd per share)	7.74	$ 7.74