Summary of significant accounting policies - Basis of Presentation - (Details) $ / shares in Units, $ in Millions		1 Months Ended
	Nov. 16, 2015 shares	Nov. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2016 shares	Dec. 31, 2015 shares
Common stock sales			25,276,258	24,930,979
IPO
Common stock sales		5,750,000
Price per share | $ / shares		$ 14.00
Net proceeds | $		$ 72.9
Reverse split ratio		4.25
Common Stock
Shares issued upon conversion of redeemable convertible preferred stock	17,647,054	17,647,054