Common stock - Narrative (Details) - USD ($)	1 Months Ended		10 Months Ended	12 Months Ended
	Apr. 30, 2014	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2015
Class of Stock [Line Items]
Common stock, shares authorized (shares)			61,000,000	100,000,000
Common stock, par value (in usd per share)			$ 0.0001	$ 0.0001
Proceeds from sale of restricted common stock to founders			$ 2,321	$ 0
Stock issued under Employee Stock Purchase Plan				0
Unvested restricted common stock
Class of Stock [Line Items]
Restricted common stock, shares sold	867,314	4,553,415
Restricted common stock, price per share (in usd per share)		$ 0.0004
Proceeds from sale of restricted common stock to founders		$ 1,950
Restricted common stock, repurchase period				90 days
Restricted common stock, repurchase price				$ 0.0004
Restricted common stock, shares outstanding				3,218,467
Restricted common stock, fair value of shares vested			$ 3,620,000	$ 15,616,000
Unvested restricted common stock | Vests upon issuance
Class of Stock [Line Items]
Restricted common stock, percent vested				25.00%
Unvested restricted common stock | Vests ratably over four years
Class of Stock [Line Items]
Restricted common stock, percent vested				75.00%
Restricted common stock, vesting period				4 years