Common stock and redeemable convertible preferred stock - Reverse Stock Split, Authorized Shares, Public Offerings (Details) $ / shares in Units, $ in Millions			12 Months Ended
	Jul. 28, 2015 USD ($) $ / shares shares	Jul. 10, 2015	Dec. 31, 2015 $ / shares shares	Dec. 31, 2016 $ / shares shares
Public offerings and related transactions
Preferred stock, authorized shares	5,000,000		5,000,000	5,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.001		$ 0.001	$ 0.001
Common stock, authorized shares	100,000,000		100,000,000	100,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001		$ 0.001	$ 0.001
Conversion of redeemable convertible preferred stock warrant into common stock warrant | Series C Warrants Issued with Senior Debt
Public offerings and related transactions
Warrants issued (in shares)	170,000
Conversion of redeemable convertible preferred stock warrant into common stock warrant | Common Stock Warrants
Public offerings and related transactions
Warrants issued (in shares)	70,833
Common Stock
Public offerings and related transactions
Reverse stock split ratio		0.4167
Common Stock | IPO
Public offerings and related transactions
Shares sold	5,520,000		5,520,000
Public offering price (in dollars per share) | $ / shares	$ 15.00
Net proceeds from issuance of stock | $	$ 75.0
Common Stock | Over-allotment option
Public offerings and related transactions
Shares sold	720,000
Common Stock | Conversion of redeemable convertible preferred stock into common stock | All series of redeemable convertible preferred stock
Public offerings and related transactions
Shares issued on conversion	9,217,983
Common Stock | Cashless exercise of redeemable convertible preferred stock warrant into common stock | Series C Warrants Issued with Series C Redeemable Preferred Stock Financing
Public offerings and related transactions
Shares issued on conversion	78,926
Shares converted	947,185