Consolidated Statement of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2010	$ 28,734	$ 34	$ 226,802	$ (198,102)
Beginning balance, shares at Dec. 31, 2010		34,017,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss and comprehensive loss	(83,903)	0	0	(83,903)
Issuance of common stock from secondary offering, net of issuance costs of $3,564	57,859	31	57,828	0
Issuance of common stock from secondary offering, net of issuance costs of $3,564, shares		30,712,000
Issuance of common stock from financing agreement at $3.86 per share, net of issuance costs of $96	1,404	0	1,404	0
Issuance of common stock from financing agreement at $3.86 per share, net of issuance costs of $96, shares		389,000
Issuance of common stock in conjunction with exercise of stock options	92	0	92	0
Issuance of common stock in conjunction with exercise of stock options, shares		67,000
Issuance of common stock from ESPP purchase	263	0	263	0
Issuance of common stock from ESPP purchase, shares		172,000
Issuance of common stock warrants	39	0	39	0
Release of restricted stock units	0	0	0	0
Release of restricted stock units, shares		12,000
Vesting of early exercised stock options	15	0	15	0
Stock-based compensation	4,809	0	4,809	0
Ending balance at Dec. 31, 2011	9,312	65	291,252	(282,005)
Ending balance, shares at Dec. 31, 2011		65,369,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss and comprehensive loss	(47,386)	0	0	(47,386)
Issuance of common stock, net of issuance costs	45,809	35	45,774	0
Issuance of common stock, net of issuance costs, shares		35,058,000
Issuance of common stock in conjunction with exercise of stock options	33	0	33	0
Issuance of common stock in conjunction with exercise of stock options, shares		18,000
Issuance of common stock from ESPP purchase	548	1	547	0
Issuance of common stock from ESPP purchase, shares		364,000
Stock-based compensation	6,157	0	6,157	0
Ending balance at Dec. 31, 2012	14,473	101	343,763	(329,391)
Ending balance, shares at Dec. 31, 2012		100,809,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss and comprehensive loss	(80,856)	0	0	(80,856)
Issuance of common stock, net of issuance costs	64,471	31	64,440	0
Issuance of common stock, net of issuance costs, shares		30,667,000
Issuance of common stock from initial public offering, net of issuance costs of $6,013	10,834	7	10,827	0
Issuance of common stock from initial public offering, net of issuance costs of $6,013, shares	6,753,104	6,753,000
Issuance of common stock in conjunction with exercise of stock options	574	0	574	0
Issuance of common stock in conjunction with exercise of stock options, shares	291,000	291,000
Issuance of common stock from ESPP purchase	385	0	385	0
Issuance of common stock from ESPP purchase, shares		304,000
Stock Issued During Period, Value, Conversion of Convertible Securities	338	0	338	0
Stock Issued During Period, Shares, Conversion of Convertible Securities		103,000
Stock-based compensation	8,006	0	8,006	0
Share-based compensation, restructuring	201		201
Ending balance at Dec. 31, 2013	$ 18,426	$ 139	$ 428,534	$ (410,247)
Ending balance, shares at Dec. 31, 2013		138,927,000