Common stock - Shelf Registration Statement and Public Offerings (Details) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended	3 Months Ended	12 Months Ended
	Nov. 08, 2018	Jul. 26, 2017	Jun. 30, 2017	Feb. 28, 2017	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Aug. 01, 2016
Public offerings and related transactions
Net proceeds from issuance of stock					$ 11
Common Stock | Underwritten Public Offering
Public offerings and related transactions
Issuance of common stock, net of issuance costs (in shares)	19,999,999		4,800,000	5,750,000
Public offering price (in dollars per share)	$ 2.30		$ 6.25	$ 5.00
Gross proceeds from issuance of stock			$ 30,000
Payments of stock issuance costs			$ 200
Term of option to purchase share			30 days
Net proceeds from issuance of stock	$ 43,400	$ 34,300		$ 26,700
Shares offered to underwriters under option granted (in shares)			720,000
Common Stock | Over-allotment option
Public offerings and related transactions
Issuance of common stock, net of issuance costs (in shares)	2,608,695			750,000
Common Stock | Sales Agreement
Public offerings and related transactions
Issuance of common stock, net of issuance costs (in shares)						651,525	749,639
Public offering price (in dollars per share)						$ 6.25	$ 5.01
Payments of stock issuance costs						$ 200	$ 100
Net proceeds from issuance of stock						$ 3,900	$ 3,600
Maximum
Public offerings and related transactions
Authorized amount to raise capital as per shelf registration statement								$ 125,000
Cowen and Company LLC | Maximum
Public offerings and related transactions
Authorized amount for issuance of common stock as per shelf registration statement								$ 40,000