Fair value measurement (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Assets:
Restricted cash	$ 545	$ 545
Recurring basis | Level 1
Assets:
Cash equivalents	10,894	23,683
Restricted cash	545	545
Total assets	11,439	24,228
Recurring basis | Level 2
Assets:
Marketable securities	37,286	32,962
Total assets	37,286	32,962
Recurring basis | Level 3
Liabilities:
Contingent consideration	1,564	1,942
Total liabilities	1,564	1,942
Recurring basis | Total
Assets:
Cash equivalents	10,894	23,683
Marketable securities	37,286	32,962
Restricted cash	545	545
Total assets	48,725	57,190
Liabilities:
Contingent consideration	1,564	1,942
Total liabilities	$ 1,564	$ 1,942