Fair value measurement (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Money market funds	$ 20,825	$ 34,340
Marketable securities	4,565	40,214
Total assets	25,390	74,554
Level 1
Assets:
Money market funds	20,825	32,843
Marketable securities	0	6,262
Total assets	20,825	39,105
Level 2
Assets:
Money market funds	0	1,497
Marketable securities	4,565	33,952
Total assets	4,565	35,449
Level 3
Assets:
Money market funds	0	0
Marketable securities	0	0
Total assets	$ 0	$ 0