Fair value measurement (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Money market accounts	$ 137,723	$ 2,255
Total assets	137,723	2,255
Liabilities:
Contingent consideration	961
Total liabilities	961
Level 1
Assets:
Money market accounts	137,723	2,255
Total assets	137,723	2,255
Liabilities:
Contingent consideration	0
Total liabilities	0
Level 2
Assets:
Money market accounts	0	0
Total assets	0	0
Liabilities:
Contingent consideration	0
Total liabilities	0
Level 3
Assets:
Money market accounts	0	0
Total assets	0	$ 0
Liabilities:
Contingent consideration	961
Total liabilities	$ 961