Fair value (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets measured at fair value
Cash and Cash Equivalents	$ 24,204	$ 9,271
Restricted Cash	375	375
Short term investments	1,104	907
Total Assets measured at fair value	25,308	10,178
Liabilities measured at fair value
Preferred Stock Warrant Liability		(109)
Total Liabilities measured at fair value		(109)
Fair Value, Inputs, Level 1 [Member]
Assets measured at fair value
Cash and Cash Equivalents	23,829	8,896
Restricted Cash	375	375
Short term investments	0
Total Assets measured at fair value	24,204	9,271
Liabilities measured at fair value
Preferred Stock Warrant Liability		0
Total Liabilities measured at fair value		0
Fair Value, Inputs, Level 2 [Member]
Assets measured at fair value
Cash and Cash Equivalents	0	0
Restricted Cash	0	0
Short term investments	1,104	907
Total Assets measured at fair value	1,104	907
Liabilities measured at fair value
Preferred Stock Warrant Liability		0
Total Liabilities measured at fair value		0
Fair Value, Inputs, Level 3 [Member]
Assets measured at fair value
Cash and Cash Equivalents	0	0
Restricted Cash	0	0
Total Assets measured at fair value	0	0
Liabilities measured at fair value
Preferred Stock Warrant Liability		(109)
Total Liabilities measured at fair value		$ (109)