Fair Value of Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2012

Fair Value of Financial Instruments

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

As of December 31, 2011 (in thousands)	Level 1		Level 2		Level 3
Assets:
Cash equivalents—U.S. treasury securities	$	35,076	$	—	$	—
Liabilities:
Convertible preferred stock warrants		—		—		1,516

As of December 31, 2012 (in thousands)	Level 1		Level 2		Level 3
Assets:
Cash equivalents—money market funds	$	25,668	$	—	$	—
Cash equivalents—certificates of deposit		—		480		—
Cash equivalents—corporate debt securities		—		5,017		—
Investments—certificates of deposit		—		240		—
Investments—commercial paper		—		12,465
Investments—corporate debt securities		—		59,533		—
Liabilities:
Derivative liability		—		—		196

Schedule of fair value of the liabilities categorized as Level 3 instruments

(in thousands)	Convertible preferred stock warrants			Derivative Liability
Balance, December 31, 2011	$	1,516		$	—
Unrealized gain included in other income (expense)		(587	)		—
Reclassification to common stock warrants		(929	)		—
Portion of convertible note allocated to derivative		—			196
Balance, December 31, 2012	$	—		$	196