Common stock - Shelf Registration Statement and offer under sales agreement (Details) - USD ($)								1 Months Ended		12 Months Ended
	Nov. 08, 2018	Nov. 05, 2018	Oct. 26, 2017	Jul. 26, 2017	Jun. 30, 2017	Feb. 17, 2017	Jul. 28, 2015	Jun. 30, 2017	Feb. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Aug. 01, 2016
Shelf registration statement
Net proceeds from issuance of stock										$ 47,292,000	$ 64,560,000	$ 13,000
Value of securities which remain available to be sold pursuant to the shelf										$ 5,200,000
Maximum
Shelf registration statement
Authorized amount to raise capital as per shelf registration statement													$ 125,000,000
Sales Agreement | Maximum | Cowen and Company LLC
Shelf registration statement
Authorized amount for issuance of common stock as per shelf registration statement													$ 40,000,000
Common Stock
Shelf registration statement
Shares issued										20,651,524	12,019,639
Stock Issued During Period, Shares, Conversion Of Restricted Stock Units										26,991
Stock Issued During Period Shares Conversion of Stock Options										832
Common Stock | Sales Agreement
Shelf registration statement
Gross proceeds from issuance of stock		$ 7,825,113
Maximum authorized amount in continuous offering		$ 7,825,113
Sales Agreement | Common Stock
Shelf registration statement
Shares issued										651,525	749,639
Sales agreement offering price (in dollars per share)										$ 6.25	$ 5.01
Gross proceeds from issuance of stock										$ 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
Public offering price (in dollars per share)										$ 6.25	$ 5.01
Underwritten Public Offering | Common Stock
Shelf registration statement
Shares issued	19,999,999				4,800,000			4,800,000	5,750,000
Sales agreement offering price (in dollars per share)	$ 2.30				$ 6.25			$ 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			$ 200,000
Public offering price (in dollars per share)	$ 2.30				$ 6.25			$ 6.25	$ 5.00
Over-allotment option | Common Stock
Shelf registration statement
Shares issued	2,608,695					750,000	720,000
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