Consolidated Statement of Stockholders' Equity (Deficit) (USD $) In Thousands, except Share data, unless otherwise specified	Total	Additional Paid-in Capital [Member]	Accumulated Deficit [Member]	Common Stock [Member]
Beginning balance at Dec. 31, 2009	$ (122,300)	$ 2,237	$ (124,538)	$ 1
Beginning balance, shares at Dec. 31, 2009				1,444,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss and comprehensive loss	(73,564)	0	(73,564)	0
Issuance of common stock from initial public offering, net of issuance costs of $6,013	51,733	51,719	0	14
Issuance of common stock from initial public offering, net of issuance costs of $6,013, shares				14,436,000
Conversion of convertible preferred stock to common stock at initial public offering	149,312	149,298	0	14
Conversion of convertible preferred stock to common stock at initial public offering, shares				14,240,000
Issuance of common stock from conversion of convertible notes, net of issuance costs of $18	15,477	15,472	0	5
Issuance of common stock from conversion of convertible notes, net of issuance costs of $18, shares				3,874,000
Beneficial conversion feature from issuance of convertible notes	4,696	4,696	0	0
Reclassification of convertible preferred stock warrants to common stock warrants	743	743	0	0
Issuance of common stock in conjunction with exercise of stock options	71	71	0	0
Vesting of early exercised stock options	59	59	0	0
Vesting of early exercised stock options, shares				23,000
Stock-based compensation	2,507	2,507	0	0
Ending balance at Dec. 31, 2010	28,734	226,802	(198,102)	34
Ending balance, shares at Dec. 31, 2010				34,017,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss and comprehensive loss	(83,903)	0	(83,903)	0
Reclassification of convertible preferred stock warrants to common stock warrants	0
Issuance of common stock from secondary offering, net of issuance costs of $3,564	57,859	57,828	0	31
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	1,404	0	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	92	0	0
Issuance of common stock in conjunction with exercise of stock options, shares				67,000
Vesting of early exercised stock options	15	15	0	0
Issuance of common stock from ESPP purchase	263	263	0	0
Issuance of common stock warrants	39	39	0	0
Issuance of common stock from ESPP purchase, shares				172,000
Release of restricted stock units	0	0	0	0
Release of restricted stock units, shares				12,000
Stock-based compensation	4,809	4,809	0	0
Ending balance at Dec. 31, 2011	9,312	291,252	(282,005)	65
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	(47,386)	0
Reclassification of convertible preferred stock warrants to common stock warrants	0
Issuance of common stock from July 2012 offering, net of issuance costs of $3,328	45,809	45,774	0	35
Issuance of common stock from July 2012 offering, net of issuance costs of $3,328, shares				35,058,000
Issuance of common stock in conjunction with exercise of stock options	33	33	0	0
Issuance of common stock in conjunction with exercise of stock options, shares	18,000			18,000
Issuance of common stock from ESPP purchase	548	547	0	1
Issuance of common stock from ESPP purchase, shares				364,000
Stock-based compensation	6,157	6,157	0	0
Ending balance at Dec. 31, 2012	$ 14,473	$ 343,763	$ (329,391)	$ 101
Ending balance, shares at Dec. 31, 2012				100,809,000