Fair Value of Financial Instruments (Tables)

6 Months Ended

Jun. 30, 2012

Fair Value of Financial Instruments [Abstract]

Fair value of financial assets and liabilities

	Fair Value Measurements as of
	June 30, 2012
	Level 1		Level 2		Level 3		Total
(in thousands)
Investments:
Money market funds	$	9,949	$	—	$	—	$	9,949
U.S. treasury notes		506		—		—		506
Commercial paper		—		4,248		—		4,248
Corporate securities		—		4,517		—		4,517
U.S. government-backed securities		—		8,319		—		8,319
Total investments	$	10,455	$	17,084	$	—	$	27,539
Convertible preferred stock warrant liability	$	—	$	—	$	235	$	235
	Fair Value Measurements as of
	December 31, 2011
(in thousands)	Level 1		Level 2		Level 3		Total
Investments:
Money market funds	$	2,973	$	—	$	—	$	2,973
U.S. treasury notes		3,051		—		—		3,051
Corporate securities		—		6,022		—		6,022
U.S. government-backed securities		—		5,436		—		5,436
Total investments	$	6,024	$	11,458	$	—	$	17,482
Convertible preferred stock warrant liability	$	—	$	—	$	117	$	117

Fair value of the convertible preferred stock warrant

The following table presents changes in financial instruments measured at fair value using Level 3 inputs:

	Convertible
	Preferred Stock
	Warrant
	Liability
	(in thousands)
Balance at December 31, 2011	$	117
Unrealized loss included in other income (expense), net		118
Balance at June 30, 2012	$	235