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, 2014 shares
Commmon stock sales				24,930,979	814,834
Reverse split ratio		4.25
Shares issued upon conversion of redeemable convertible preferred stock	17,647,054		17,647,054
IPO
Commmon stock sales	5,750,000
Price per share | $ / shares	$ 14.00
Estimated net proceeds | $	$ 72.9
Common Stock
Shares issued upon conversion of redeemable convertible preferred stock				17,647,054