Fair value of financial instruments Reconciliation of preferred stock warrants (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Exercise of preferred stock warrants			$ 0	$ (2)
Convertible Preferred Stock Warrant
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period	3,216	1,593	1,853	1,127
Change in fair value	(75)	735	1,631	1,201
Exercise of preferred stock warrants	0	(255)	(343)	(255)
Conversion of preferred stock warrants to common stock warrants	(3,141)	0	(3,141)	0
Fair value at end of period	$ 0	$ 2,073	$ 0	$ 2,073