Note Payable (Details) (USD $)	12 Months Ended				0 Months Ended		12 Months Ended		20 Months Ended		11 Months Ended	12 Months Ended		0 Months Ended		1 Months Ended	12 Months Ended			0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Common Stock	Feb. 28, 2013 Subsequent event	Jan. 31, 2013 Subsequent event	Dec. 31, 2012 Series C redeemable convertible preferred stock warrants	Dec. 31, 2011 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	Oct. 31, 2012 Series D redeemable convertible preferred stock warrants	Dec. 31, 2011 Series D redeemable convertible preferred stock warrants	Jul. 31, 2012 Series D redeemable convertible preferred stock warrants	Oct. 02, 2012 Senior loan financing	Jul. 23, 2012 Senior loan financing	Nov. 30, 2012 Senior loan financing	Dec. 31, 2012 Senior loan financing	Aug. 01, 2012 Senior loan financing	Mar. 21, 2011 Senior loan financing	Feb. 28, 2013 Senior loan financing Subsequent event	Jan. 31, 2013 Senior loan financing Subsequent event
Credit Facility
Maximum borrowing capacity																			$ 15,000,000
Remaining borrowing capacity	10,000,000
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%
Amount drawn					5,000,000	5,000,000								5,000,000						5,000,000	5,000,000
Amount that the entity can draw, pursuant to the extension obtained																10,000,000
Exercise price (in dollars per unit)										$ 13.3530			$ 13.9040
Estimated fair value of warrant										345,000			1,074,000
Risk-free interest rate (as a percent)								1.75%				1.89%
Volatility rate (as a percent)								80.00%				81.00%
Warrants issued (in shares)				120,000						34,000			86,000
Increase (decrease) in fair value									55,000		18,000
Warrant and other interest income (expense), net	(861,000)	(304,000)	589,000				91,000	36,000
Stock exercised upon issuance of warrants on cashless basis				49,000
Amortization of debt issuance costs	272,000
Interest expense related to the note payable	169,000
Accrued interest expense	22,000
Future maturities of the Note payable
2013	1,500,000
2014	2,000,000
2015	1,500,000
Total	5,000,000
Unamortized debt issuance cost	(1,076,000)
Note payable excluding unamortized debt issuance cost	3,924,000
Current portion	(1,160,000)
Note payable - net of current portion	$ 2,764,000