Consolidated Statements of Stockholders' Equity (USD $) In Thousands, except Share data	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2010	$ 29,688	$ 4	$ 353,278	$ (324,106)	$ 512
Beginning balance (in shares) at Dec. 31, 2010		39,350,561
Issuance of common stock and warrants, net of issuance costs (in shares)		12,565,446
Issuance of common stock and warrants, net of issuance costs	13,900	1	13,899
Issuance of common stock from exercise of stock options (in shares)		10,434
Issuance of common stock from exercise of stock options	13		13
Issuance of common stock pursuant to employee stock purchase plan (in shares)	189,879	189,879
Issuance of common stock pursuant to employee stock purchase plan	109		109
Issuance of common stock in connection with lease termination (in shares)		782,339
Issuance of common stock in connection with lease termination	1,322		1,322
Net loss	(22,765)			(22,765)
Stock-based compensation	1,598		1,598
Other comprehensive income (loss)	(503)				(503)
Ending balance at Dec. 31, 2011	23,362	5	370,219	(346,871)	9
Ending balance (in shares) at Dec. 31, 2011		52,898,659
Issuance of common stock and warrants, net of issuance costs (in shares)		19,000,000
Issuance of common stock and warrants, net of issuance costs	80,538	2	80,536
Issuance of common stock from exercise of stock options (in shares)		293,595
Issuance of common stock from exercise of stock options	453		453
Issuance of common stock pursuant to employee stock purchase plan (in shares)	205,862	205,862
Issuance of common stock pursuant to employee stock purchase plan	123		123
Issuance of common stock from exercise of warrants on a net issuance basis (in shares)	874,719	936,100
Issuance of common stock from exercise of warrants on a net issuance basis
Issuance of common stock under ATM Agreement, net of issuance costs (in shares)		5,347,137
Issuance of common stock under ATM Agreement, net of issuance costs	17,090	1	17,089
Reclassification to redeemable common stock (in shares)		(5,347,137)
Reclassification to redeemable common stock	(17,658)	(1)	(17,657)
Net loss	(20,849)			(20,849)
Stock-based compensation	1,930		1,930
Other comprehensive income (loss)	(5)				(5)
Ending balance at Dec. 31, 2012	84,984	7	452,693	(367,720)	4
Ending balance (in shares) at Dec. 31, 2012		73,334,216
Issuance of common stock in public offering, net of issuance costs (in shares)		9,200,000
Issuance of common stock in public offering, net of issuance costs	107,883	1	107,882
Issuance of common stock from exercise of stock options (in shares)	1,455,406	1,455,406
Issuance of common stock from exercise of stock options	3,441		3,441
Issuance of common stock pursuant to employee stock purchase plan (in shares)	122,853	122,853
Issuance of common stock pursuant to employee stock purchase plan	358		358
Issuance of common stock from exercise of warrants on a net issuance basis (in shares)	1,643,006	1,643,006
Issuance of common stock from exercise of warrants on a net issuance basis
Reclassification from redeemable common stock (in shares)		5,347,137
Reclassification from redeemable common stock	17,658	1	17,657
Net loss	(37,948)			(37,948)
Stock-based compensation	5,711		5,711
Other comprehensive income (loss)	44				44
Ending balance at Dec. 31, 2013	$ 182,131	$ 9	$ 587,742	$ (405,668)	$ 48
Ending balance (in shares) at Dec. 31, 2013		91,102,618