Common stock and redeemable convertible preferred stock (Details 2) $ / shares in Units, $ in Millions	Jul. 28, 2015 USD ($) $ / shares shares	Jul. 10, 2015	Jun. 30, 2015 $ / shares shares	Dec. 31, 2014 $ / shares shares
Public offerings and related transactions
Common stock, authorized shares			35,000,000	35,000,000
Common stock, par value (in dollars per share) | $ / shares			$ 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 Warrant
Public offerings and related transactions
Warrants issued (in shares)	70,833
Common Stock | IPO
Public offerings and related transactions
Shares sold	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 converted	947,185
Shares issued on conversion	78,926
Subsequent event
Public offerings and related transactions
Preferred stock, authorized shares	5,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.001
Common stock, authorized shares	100,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001
Subsequent event | Common Stock
Public offerings and related transactions
Reverse stock split ratio		0.4167