Common stock - Offer under sales agreement (Details) - USD ($)						1 Months Ended		12 Months Ended
	Nov. 08, 2018	Nov. 05, 2018	Oct. 26, 2017	Jul. 26, 2017	Feb. 17, 2017	Jun. 30, 2017	Feb. 28, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 18, 2019
Shelf registration statement
Net proceeds from issuance of stock								$ 11,000	$ 47,292,000	$ 64,560,000
Cantor Sales Agreement | Maximum | Cantor Fitzgerald & Co
Shelf registration statement
Authorized amount for issuance of common stock as per second shelf registration statement											$ 30,000,000
Value of common stock which remains available to be sold under the sales agreement								$ 30,000,000
Common Stock
Shelf registration statement
Issuance of common stock, net of issuance costs (in shares)									20,651,524	12,019,639
Issuance of common stock upon RSU conversion								20,328	26,991
Shares issued upon the exercise of outstanding stock options								36,040	832	1,249
Common Stock | Cantor Sales Agreement
Shelf registration statement
Issuance of common stock, net of issuance costs (in shares)								0
Sales Agreement | Common Stock
Shelf registration statement
Issuance of common stock, net of issuance costs (in shares)									651,525	749,639
Sales agreement offering price (in dollars per share)									$ 6.25	$ 5.01
Gross proceeds from issuance of stock		$ 7,825,113							$ 4,100,000	$ 3,700,000
Net proceeds from issuance of stock									3,900,000	3,600,000
Payments of stock issuance costs									$ 200,000	$ 100,000
Maximum authorized amount in continuous offering		$ 7,825,113
Underwritten Public Offering | Common Stock
Shelf registration statement
Issuance of common stock, net of issuance costs (in shares)	19,999,999					4,800,000	5,750,000
Sales agreement offering price (in dollars per share)	$ 2.30					$ 6.25	$ 5.00
Net proceeds from issuance of stock	$ 43,400,000			$ 34,300,000			$ 26,700,000
Payments of stock issuance costs						$ 200,000
Over-allotment option | Common Stock
Shelf registration statement
Issuance of common stock, net of issuance costs (in shares)	2,608,695				750,000
2019 Shelf
Shelf registration statement
Value of securities which remain available to be sold pursuant to the shelf								$ 100,000,000
2019 Shelf | Maximum
Shelf registration statement
Authorized amount to raise capital as per second shelf registration statement											$ 100,000,000
Conversion Of Convertible Notes | Senior Secured Convertible Notes
Shelf registration statement
Amount of debt converted into stock			$ 6,600,000
Conversion price			$ 7.08
Conversion price, expressed as a percentage of stock price			95.00%
Number of trading days used to determine conversion stock price			3 days
Number of shares of stock into which debt was converted			929,967