Fair value of financial instruments Reconciliation of preferred stock warrants (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 31, 2012	Mar. 31, 2011
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Exercise of preferred stock warrants			$ 0	$ 0
Convertible Preferred Stock Warrant
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value at beginning of period			1,853	1,127	0	2,073
Change in fair value	0	(286)	1,631	915
Exercise of preferred stock warrants	0	0	(343)	(255)
Conversion of preferred stock warrants to common stock warrants	0	0	(3,141)	0
Fair value at end of period	$ 0	$ 1,787	$ 0	$ 1,787	$ 0	$ 2,073