Note Payable (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended	3 Months Ended	6 Months Ended	20 Months Ended		3 Months Ended	6 Months Ended	11 Months Ended		0 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2012 Series C redeemable convertible preferred stock warrants	Jun. 30, 2012 Series C redeemable convertible preferred stock warrants	Oct. 31, 2012 Series C redeemable convertible preferred stock warrants	Mar. 21, 2011 Series C redeemable convertible preferred stock warrants	Jun. 30, 2012 Series D redeemable convertible preferred stock warrants	Jun. 30, 2012 Series D redeemable convertible preferred stock warrants	Oct. 31, 2012 Series D redeemable convertible preferred stock warrants	Dec. 31, 2011 Series D redeemable convertible preferred stock warrants	Jul. 23, 2012 Senior loan financing	Jun. 30, 2013 Senior loan financing	Aug. 01, 2012 Senior loan financing	Mar. 21, 2011 Senior loan financing
Note Payable
Maximum borrowing capacity																	$ 15,000,000
Drawn down limit to extend drawdown period																5,000,000
Additional extension to the initial drawdown date (in days)														60 days
Initial period of interest only payments															6 months
Fixed rate of interest in initial period (as a percent)															11.50%
Period of payment of interest and principal following the initial period															30 months
Fixed rate of interest in period following initial period (as a percent)															8.50%
Final payment percentage															6.00%
Warrants issued (in shares)									34,000				86,000
Exercise price (in dollars per unit)									$ 13.3530				$ 13.9040
Estimated fair value of warrants									345,000				1,074,000
Amount drawn under credit facility	15,000,000		15,000,000		5,000,000
Dividend yield (as a percent)							0.00%				0.00%
Risk-free interest rate (as a percent)							1.43%				1.58%
Volatility rate (as a percent)							76.00%				77.00%
Increase (decrease) in fair value								55,000				18,000
Warrant and other income (expense), net		(577,000)		(476,000)		(105,000)	(97,000)			(5,000)	2,000
Stock exercised upon issuance of warrants on cashless basis					49,000
Warrants outstanding (in shares)	0		0		178
Amortization of debt issuance costs	85,000	52,000	170,000	104,000
Interest expense related to the note payable	469,000	0	779,000	0
Amount drawn under credit facility		0		0
Remaining borrowing capacity	0		0
Accrued interest expense	120,000		120,000		22,000
Unamortized debt issuance cost	$ 906,000		$ 906,000