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	$ 0	$ 100	$ 386,511	$ 3,603	$ (340,831)
Beginning Balance (in shares) at Dec. 31, 2013		0	100,159,618
Warrant exercise (in shares)			21,977
Warrant exercise				62	(62)
Stock-based compensation	2,435			2,435
Exercise of employee stock options (in shares)	583,038		583,038
Exercise of employee stock options	1,317		1	1,316
Cancelled restricted stock (in shares)			(8,936)
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	(15,287)					(15,287)
Ending Balance at Jun. 30, 2014	$ 37,716	$ 0	$ 101	$ 390,192	$ 3,541	$ (356,118)
Ending Balance (in shares) at Jun. 30, 2014		0	100,725,066