Statement of Stockholders' Equity (USD $) In Thousands, except Share data	Total	Preferred Stock	Common Stock	Additional Paid-in Capital Common Stock	Additional Paid-in Capital Warrants	Deficit Accumulated During the Development Stage
Beginning Balance at Dec. 31, 2013	$ 49,383		$ 100	$ 386,511	$ 3,603	$ (340,831)
Beginning Balance (in shares) at Dec. 31, 2013			100,159,618
Stock-based compensation	1,223			1,223
Exercise of employee stock options (in shares)	479,705		479,705
Exercise of employee stock options	969		1	968
Cancelled restricted stock (in shares)			(8,026)
Cancelled restricted stock
Repurchase of shares of restricted common stock (in shares)			(30,631)
Repurchase of shares of restricted common stock	(132)			(132)
Net loss	(9,711)					(9,711)
Ending Balance at Mar. 31, 2014	$ 41,732		$ 101	$ 388,570	$ 3,603	$ (350,542)
Ending Balance (in shares) at Mar. 31, 2014			100,600,666