Fair Value of Financial Instruments (Tables)
 12 Months Ended
Dec. 31, 2013
Fair Value Disclosures [Abstract]  
Schedule of the entity's financial assets and liabilities at fair value
The following tables set forth the fair value of the Company’s cash equivalents and short-term investments by level within the fair value hierarchy (in thousands):
 
As of December 31, 2013
 
Fair Value
 
Level I
 
Level II
 
Level III
Assets:
 
 
 
 
 
 
 
Money market funds
$
49,117

 
$
49,117

 
$
—

 
$
—

Total assets measured at fair value
$
49,117

 
$
49,117

 
$
—

 
$
—

 
As of December 31, 2012
 
Fair Value
 
Level I
 
Level II
 
Level III
Assets:
 
 
 
 
 
 
 
Money market funds
$
70,488

 
$
70,488

 
$
—

 
$
—

Total assets measured at fair value
$
70,488

 
$
70,488

 
$
—

 
$
—
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation
The following table sets forth a summary of the changes in the fair value of the Company's Level III financial liabilities (in thousands):
 
Years Ended December 31,
 
2013
 
2012
 
2011
Fair value—beginning of period
$
—

 
$
—

 
$
215

Initial fair value of derivative liability
—

 
—

 
2,842

Mark-to-market of warrant and derivative liabilities
—

 
—

 
25,030

Reclassification of warrant liability to equity
—

 
—

 
(769
)
Reclassification of derivative liability to note payable
—

 
—

 
(27,318
)
Fair value—end of period
$
—

 
$
—

 
$
—