Summary of significant accounting policies - Basis of Presentation - (Details) $ / shares in Units, $ in Millions			1 Months Ended	12 Months Ended
	Nov. 16, 2015 USD ($) $ / shares shares	Oct. 29, 2015	Nov. 30, 2015 shares	Dec. 31, 2015 shares	Dec. 31, 2016 shares
Common stock sales				24,930,979	25,597,912
Reverse split ratio		4.25
Shares issued upon conversion of redeemable convertible preferred stock	17,647,054		17,647,054
IPO
Common stock sales	5,750,000
Price per share | $ / shares	$ 14.00
Net proceeds | $	$ 72.9
Common Stock
Shares issued upon conversion of redeemable convertible preferred stock				17,647,054