Fair value measurement (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Level 1
Assets:
Money market accounts	$ 5,602	$ 23,683
Total assets	5,602	23,683
Level 2
Assets:
Marketable securities	1,614	32,962
Total assets	1,614	32,962
Level 3
Liabilities:
Contingent consideration	(131)	(1,942)
Total liabilities	(131)	(1,942)
Total
Assets:
Money market accounts	5,602	23,683
Marketable securities	1,614	32,962
Total assets	7,216	56,645
Liabilities:
Contingent consideration	(131)	(1,942)
Total liabilities	$ (131)	$ (1,942)