Fair value of financial instruments (Tables)

9 Months Ended

Sep. 30, 2012

Fair Value Disclosures [Abstract]

Schedule of assets and liabilities measured at fair value on a recurring basis, by level

The Company’s assets and liabilities that are measured at fair value on a recurring basis, by level, within the fair value hierarchy as of September 30, 2012 and December 31, 2011, are summarized as follows (in thousands):

	September 30, 2012								December 31, 2011
	Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3		Total
Assets
Money market funds	$	21,509	$	—	$	—	$	21,509	$	—	$	—	$	—	$	—
Certificates of deposits	996		—		—		996		—		—		—		—
Commercial paper	62,362		—		—		62,362		—		—		—		—
U.S. government and agency securities	9,997		—		—		9,997		—		—		—		—
Total assets measured at fair value	$	94,864	$	—	$	—	$	94,864	$	—	$	—	$	—	$	—
Liabilities
Convertible preferred stock warrants	$	—	$	—	$	—	$	—	$	—	$	—	$	1,853	$	1,853
Total liabilities measured at fair value	$	—	$	—	$	—	$	—	$	—	$	—	$	1,853	$	1,853

Reconcilation of preferred stock warrants

The following table presents a reconciliation of the preferred stock warrants measured and recorded at fair value on a recurring basis, using significant unobservable inputs (Level 3) for the three months and nine months ended September 30, 2012 and 2011, respectively:

	Three months ended September 30,					Nine months ended September 30,
(in thousands)	2012		2011			2012			2011
Fair value at beginning of period	$	—	$	2,073		$	1,853		$	1,127
Change in fair value	—		(286		)	1,631			915
Exercise of preferred stock warrants	—		—			(343		)	(255		)
Conversion of preferred stock warrants to common stock warrants	—		—			(3,141		)	—
Fair value at end of period	$	—	$	1,787		$	—		$	1,787