Fair value measurement (Details) - Recurring basis - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Money market accounts	$ 141,204	$ 34,340
Marketable securities	29,301	40,214
Total assets	170,505	74,554
Level 1
Assets:
Money market accounts	141,204	32,843
Marketable securities	3,776	6,262
Total assets	144,980	39,105
Level 2
Assets:
Money market accounts	0	1,497
Marketable securities	25,525	33,952
Total assets	25,525	35,449
Level 3
Assets:
Money market accounts	0	0
Marketable securities	0	0
Total assets	$ 0	$ 0