Fair value measurement (Details) - Recurring basis - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets:
Money market accounts	$ 32,519	$ 34,340
Marketable securities	7,580	40,214
Total assets	40,099	74,554
Level 1
Assets:
Money market accounts	32,519	32,843
Marketable securities	0	6,262
Total assets	32,519	39,105
Level 2
Assets:
Money market accounts	0	1,497
Marketable securities	7,580	33,952
Total assets	7,580	$ 35,449
Level 3
Assets:
Money market accounts	0
Marketable securities	0
Total assets	$ 0