Cash and cash equivalents and marketable securities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Cash and cash equivalents:
Cash	$ 15,206		$ 15,206		$ 6,061
Money market accounts	136,741		136,741		34,340
Total cash and cash equivalents	151,947		151,947		40,401
Short-term marketable securities-available-for-sale:
Marketable securities	20,100		20,100		40,214
Total short-term marketable securities	20,100		20,100		$ 40,214
Interest income	$ 172	$ 540	$ 468	$ 1,254