Cash, cash equivalents and short-term investments (Details) (USD $)	3 Months Ended								3 Months Ended		36 Months Ended
	Mar. 31, 2014 item	Dec. 31, 2013 item	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2014 Corporate debt securities due in one year or less	Dec. 31, 2013 Corporate debt securities due in one year or less	Mar. 31, 2014 Corporate debt securities due in two years or less	Dec. 31, 2013 Corporate debt securities due in two years or less	Mar. 31, 2014 Short-term investments	Mar. 31, 2013 Short-term investments	Mar. 31, 2014 Short-term investments	Dec. 31, 2013 Short-term investments	Mar. 31, 2014 Financial institution debt securities and other corporate debt securities located in Canada, the United Kingdom, the Netherlands, Australia, France and Norway	Dec. 31, 2013 Financial institution debt securities and other corporate debt securities located in Canada, the United Kingdom, the Netherlands, Australia, France and Norway
Cash, cash equivalents and marketable securities
Cash and money market funds, Amortized Cost	$ 56,134,000	$ 18,078,000
Cash and money market funds, Fair Value	56,134,000	18,078,000
Amortized Cost	89,465,000	44,417,000			26,058,000	22,631,000	7,273,000	3,708,000	33,331,000		33,331,000	26,339,000	9,300,000	11,700,000
Gross Unrealized Gains	9,000	13,000			9,000	11,000		2,000	9,000		9,000	13,000
Gross Unrealized Losses	(12,000)	(3,000)			(8,000)	(2,000)	(4,000)	(1,000)	(12,000)		(12,000)	(3,000)
Fair Value	89,462,000	44,427,000			26,059,000	22,640,000	7,269,000	3,709,000	33,328,000		33,328,000	26,349,000
Amortized cost of cash and cash equivalents and marketable securities	89,465,000	44,417,000
Fair value of cash and cash equivalents and marketable securities	89,462,000	44,427,000
Amortized cost of cash and cash equivalents	56,134,000	18,078,000	43,188,000	14,776,000
Fair value of cash and cash equivalents	56,134,000	18,078,000
Number of debt securities that had been in an unrealized loss position for less than 12 months	10	8
Number of debt securities that had been in an unrealized loss position for 12 months or longer	0
Fair value of debt securities that had been in an unrealized loss position for less than 12 months	13,900,000	9,500,000
Realized gains										0
Realized losses									0	0	0
Other-than-temporary impairments recognized									$ 0	$ 0	$ 0