Common stock - Public Offerings (Details) $ / shares in Units, $ in Thousands						1 Months Ended		12 Months Ended
	Nov. 08, 2018 USD ($) $ / shares shares	Jul. 26, 2017 USD ($)	Jun. 30, 2017 USD ($) $ / shares shares	Feb. 17, 2017 USD ($) shares	Jul. 28, 2015 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Feb. 28, 2017 $ / shares shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)
Public offerings and related transactions
Net proceeds from issuance of stock | $								$ 47,292	$ 64,560	$ 13
Reverse stock split ratio					0.4167
Common Stock
Public offerings and related transactions
Issuance of common stock, net of issuance costs (in shares) | shares								20,651,524	12,019,639
Common Stock | IPO
Public offerings and related transactions
Issuance of common stock, net of issuance costs (in shares) | shares					5,520,000
Public offering price (in dollars per share) | $ / shares					$ 15.00
Net proceeds from issuance of stock | $					$ 75,000
Common Stock | Underwritten Public Offering
Public offerings and related transactions
Issuance of common stock, net of issuance costs (in shares) | shares	19,999,999		4,800,000			4,800,000	5,750,000
Public offering price (in dollars per share) | $ / shares	$ 2.30		$ 6.25			$ 6.25	$ 5.00
Gross proceeds from issuance of stock | $			$ 30,000			$ 30,000
Payments of stock issuance costs | $			$ 200			$ 200
Term of option to purchase share			30 days			30 days
Net proceeds from issuance of stock | $	$ 43,400	$ 34,300		$ 26,700
Shares offered to underwriters under option granted (in shares) | shares			720,000			720,000
Common Stock | Over-allotment option
Public offerings and related transactions
Issuance of common stock, net of issuance costs (in shares) | shares	2,608,695			750,000	720,000