Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (USD $) In Thousands, except Share data, unless otherwise specified	Total	Preferred Stock Redeemable Convertible Preferred Stock	Preferred Stock Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Deficit Accumulated During the Development Stage
Balance at Jan. 05, 2005
Increase (Decrease) in Stockholders' Equity
Sale of stock, net of expenses	$ 216				$ 216
Sale of stock, net of expenses (in shares)				338,116
Net loss	(1,067)					(1,067)
Balance at Dec. 31, 2005	(851)				216	(1,067)
Balance (in shares) at Dec. 31, 2005				338,116
Increase (Decrease) in Stockholders' Equity
Stock-based compensation	44				44
Stock-based compensation (in shares)				114,158
Conversion of convertible notes and accrued interest into Series A redeemable convertible preferred stock	648	2,590			648
Conversion of convertible notes and accrued interest into Series A redeemable convertible preferred stock (in shares)		3,481,645
Sale of Series A redeemable convertible preferred stock at $0.93 per share for the years 2006, 2007, and 2008 and net of expenses of $267 and $20 for the years 2006 and 2007, respectively		7,233
Sale of Series A redeemable convertible preferred stock at $0.93 per share for the year 2006, 2007, and 2008 and net of expenses of $267 and $20 for the year 2006 and 2007, respectively (in shares)		8,064,516
Accretion of Series A and Series B redeemable convertible preferred stock to redemption value	(341)	341			(341)
Net loss	(5,215)					(5,215)
Balance at Dec. 31, 2006	(5,715)	10,164			567	(6,282)
Balance (in shares) at Dec. 31, 2006		11,546,161		452,274
Increase (Decrease) in Stockholders' Equity
Stock-based compensation	59				59
Sale of Series A redeemable convertible preferred stock at $0.93 per share for the years 2006, 2007, and 2008 and net of expenses of $267 and $20 for the years 2006 and 2007, respectively		4,980
Sale of Series A redeemable convertible preferred stock at $0.93 per share for the year 2006, 2007, and 2008 and net of expenses of $267 and $20 for the year 2006 and 2007, respectively (in shares)		5,376,345
Accretion of Series A and Series B redeemable convertible preferred stock to redemption value	(1,126)	1,126			(626)	(500)
Net loss	(9,675)					(9,675)
Balance at Dec. 31, 2007	(16,457)	16,270				(16,457)
Balance (in shares) at Dec. 31, 2007		16,922,506		452,274
Increase (Decrease) in Stockholders' Equity
Stock-based compensation	158				158
Exercise of stock options (in shares)				155
Sale of Series A redeemable convertible preferred stock at $0.93 per share for the years 2006, 2007, and 2008 and net of expenses of $267 and $20 for the years 2006 and 2007, respectively		2,500
Sale of Series A redeemable convertible preferred stock at $0.93 per share for the year 2006, 2007, and 2008 and net of expenses of $267 and $20 for the year 2006 and 2007, respectively (in shares)		2,688,171
Sale of Series B redeemable convertible preferred stock at $0.93 per share for the years 2008 and 2009 and net of expenses of $304 and $16 for the years 2008 and 2009, respectively		20,708
Sale of Series B redeemable convertible preferred stock at $0.93 per share for the years 2008 and 2009 and net of expenses of $304 and $16 for the years 2008 and 2009, respectively (in shares)		22,594,385
Accretion of Series A and Series B redeemable convertible preferred stock to redemption value	(2,330)	2,330			(158)	(2,172)
Net loss	(17,511)					(17,511)
Balance at Dec. 31, 2008	(36,140)	41,808				(36,140)
Balance (in shares) at Dec. 31, 2008		42,205,062		452,429,000
Increase (Decrease) in Stockholders' Equity
Stock-based compensation	319				319
Forfeiture/Cancellation of restricted stock (in shares)				(61,753)
Sale of Series B redeemable convertible preferred stock at $0.93 per share for the years 2008 and 2009 and net of expenses of $304 and $16 for the years 2008 and 2009, respectively		8,155
Sale of Series B redeemable convertible preferred stock at $0.93 per share for the years 2008 and 2009 and net of expenses of $304 and $16 for the years 2008 and 2009, respectively (in shares)		8,786,952
Conversion of convertible notes and accrued interest into Series B redeemable convertible preferred stock		1,957
Conversion of convertible notes and accrued interest into Series B redeemable convertible preferred stock (in shares)		2,104,326
Accretion of Series A and Series B redeemable convertible preferred stock to redemption value	(3,616)	3,617			(875)	(2,741)
Beneficial conversion feature related to convertible notes and warrant agreements	556				556
Net loss	(15,591)					(15,591)
Balance at Dec. 31, 2009	(54,472)	55,537				(54,472)
Balance (in shares) at Dec. 31, 2009		53,096,340		390,676
Increase (Decrease) in Stockholders' Equity
Stock-based compensation	543				543
Exercise of stock options	8				8
Exercise of stock options (in shares)				4,878
Accretion of Series A and Series B redeemable convertible preferred stock to redemption value	(2,536)	2,534			(1,568)	(968)
Sale of stock, net of expenses	42,972				42,967
Sale of stock, net of expenses (in shares)				5,000,000
Common shares issued pursuant to conversion of preferred	58,072	(58,071)		8	58,064
Common shares issued pursuant to conversion of preferred (in shares)		(53,096,340)		7,861,785
Conversion of convertible notes and accrued interest into common stock	10,338			2	10,336
Conversion of convertible notes and accrued interest into common stock (in shares)				1,292,122
Beneficial conversion feature related to convertible notes and warrant agreements	2,584				2,584
Reclassification of warrants to purchase common stock	1,113				1,113
Net loss	(24,357)					(24,357)
Balance at Dec. 31, 2010	34,265			15	114,047	(79,797)
Balance (in shares) at Dec. 31, 2010				14,549,461
Increase (Decrease) in Stockholders' Equity
Stock-based compensation	1,216				1,216
Issuance of restricted stock (in shares)				16,000
Exercise of stock options	51				51
Exercise of stock options (in shares)				27,534
Sale of stock, net of expenses	354				354
Sale of stock, net of expenses (in shares)				155,587
Fair value of warrants issued in connection with loan facility	272				272
Net loss	(23,187)					(23,187)
Balance at Dec. 31, 2011	12,971			15	115,940	(102,984)
Balance (in shares) at Dec. 31, 2011				14,748,582
Increase (Decrease) in Stockholders' Equity
Stock-based compensation	2,623				2,623
Forfeiture/Cancellation of restricted stock (in shares)				(11,000)
Exercise of stock options	57				57
Exercise of stock options (in shares)				40,367
Sale of stock, net of expenses	11,537		11,537
Sale of stock, net of expenses (in shares)			14,000
Common shares issued pursuant to conversion of preferred			(4,282)	5	4,277
Common shares issued pursuant to conversion of preferred (in shares)	5,196,000		(5,196)	5,196,000
Common shares issued pursuant to loan agreement	146				146
Common shares issued pursuant to loan agreement (in shares)				50,000
Deemed dividend on preferred shares					13,250	(13,250)
Fair value of warrants issued in connection with loan facility	213				213
Net loss	(24,534)					(24,534)
Balance at Dec. 31, 2012	$ 3,013		$ 7,255	$ 20	$ 136,506	$ (140,768)
Balance (in shares) at Dec. 31, 2012			8,804	20,023,949
Increase (Decrease) in Stockholders' Equity
Common shares issued pursuant to conversion of preferred (in shares)				8,804,000
Balance at Mar. 31, 2013