Cash and cash equivalents and marketable securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Cash and cash equivalents:
Cash	$ 14,453		$ 14,453		$ 6,061
Money market accounts	32,519		32,519		34,340
Total cash and cash equivalents	46,972		46,972		40,401
Short-term marketable securities-available-for-sale:
Marketable securities	7,580		7,580		40,214
Total short-term marketable securities	7,580		7,580		$ 40,214
Interest income	$ 81	$ 431	$ 549	$ 1,685