Cash and cash equivalents and marketable securities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Cash and cash equivalents:
Cash	$ 16,677	$ 12,851
Money market accounts	39,001	81,082
Marketable securities	2,460
Total cash and cash equivalents	58,138	93,933	25,721	22,629
Short-term marketable securities available-for-sale:
Marketable securities	41,558
Total short-term marketable securities	41,558	0
Interest income	$ 183	$ 37	$ 31