Fair value measurement (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Assets:
Restricted cash	$ 30	$ 30
Recurring basis | Level 1
Assets:
Cash equivalents	37,831	39,001
Restricted cash	30	30
Total assets	37,861	39,031
Recurring basis | Level 2
Assets:
Cash equivalents		2,460
Marketable securities	45,224	41,558
Total assets	45,224	44,018
Recurring basis | Level 3
Liabilities:
Contingent consideration	(1,329)
Total liabilities	(1,329)
Recurring basis | Total
Assets:
Cash equivalents	37,831	41,461
Marketable securities	45,224	41,558
Restricted cash	30	30
Total assets	83,085	83,049
Liabilities:
Contingent consideration	(1,329)
Total liabilities	$ (1,329)