Fair value measurement (Details) - Recurring basis - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Money market accounts	$ 45,791	$ 137,723
Marketable securities	36,594
Total assets	82,385	137,723
Liabilities:
Contingent consideration		961
Total liabilities		961
Level 1
Assets:
Money market accounts	45,791	137,723
Marketable securities	4,250
Total assets	50,041	137,723
Level 2
Assets:
Money market accounts	0
Marketable securities	32,344
Total assets	32,344
Level 3
Assets:
Money market accounts	0
Marketable securities	0
Total assets	$ 0
Liabilities:
Contingent consideration		961
Total liabilities		$ 961