Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Total
Balance at Dec. 31, 2012	$ 639	$ 372,123	$ 78	$ (274,245)	$ 98,595
Balance (in shares) at Dec. 31, 2012	63,909,800
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs of $4.4, $11.0 and $4.8 million for 2015, 2014 and 2013, respectively	$ 63	73,232			73,296
Issuance of common stock, net of issuance costs (in shares)	6,325,000
Exercise of common stock options and warrants	$ 4	1,337			1,341
Exercise of common stock options and warrants (in shares)	418,146
Issuance of common stock to fund the Company's 401(k) match	$ 1	354			354
Issuance of common stock to fund the Company's 401(k) match (in shares)	30,538
Compensation expense related to issuance of stock options and restricted stock awards	$ 3	7,741			7,744
Compensation expense related to issuance of stock options and restricted stock awards (in shares)	328,422
Purchase of employee stock purchase plan shares	$ 1	452			453
Purchase of employee stock purchase plan shares (in shares)	59,932
Net loss				(46,514)	(46,514)
Accumulated other comprehensive income			47		47
Balance at Dec. 31, 2013	$ 711	455,239	125	(320,759)	135,316
Balance (in shares) at Dec. 31, 2013	71,071,838
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs of $4.4, $11.0 and $4.8 million for 2015, 2014 and 2013, respectively	$ 155	238,425			238,580
Issuance of common stock, net of issuance costs (in shares)	15,500,000
Exercise of common stock options and warrants	$ 15	2,625			2,640
Exercise of common stock options and warrants (in shares)	1,522,753
Issuance of common stock to fund the Company's 401(k) match	$ 1	455			456
Issuance of common stock to fund the Company's 401(k) match (in shares)	32,666
Compensation expense related to issuance of stock options and restricted stock awards	$ 4	11,516			11,520
Compensation expense related to issuance of stock options and restricted stock awards (in shares)	410,619
Purchase of employee stock purchase plan shares	$ 1	759			760
Purchase of employee stock purchase plan shares (in shares)	88,166
Net loss				(100,048)	(100,048)
Accumulated other comprehensive income			(240)		(240)
Balance at Dec. 31, 2014	$ 887	709,019	(115)	(420,807)	$ 288,984
Balance (in shares) at Dec. 31, 2014	88,626,042				88,626,042
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs of $4.4, $11.0 and $4.8 million for 2015, 2014 and 2013, respectively	$ 70	174,070			$ 174,140
Issuance of common stock, net of issuance costs (in shares)	7,000,000
Exercise of common stock options and warrants	$ 3	1,245			1,248
Exercise of common stock options and warrants (in shares)	281,315
Issuance of common stock to fund the Company's 401(k) match		836			836
Issuance of common stock to fund the Company's 401(k) match (in shares)	21,826
Compensation expense related to issuance of stock options and restricted stock awards	$ 6	18,044			18,050
Compensation expense related to issuance of stock options and restricted stock awards (in shares)	568,818
Purchase of employee stock purchase plan shares	$ 2	1,717			1,719
Purchase of employee stock purchase plan shares (in shares)	176,785
Net loss				(157,803)	(157,803)
Accumulated other comprehensive income			(318)		(318)
Balance at Dec. 31, 2015	$ 968	$ 904,931	$ (433)	$ (578,610)	$ 326,856
Balance (in shares) at Dec. 31, 2015	96,674,786				96,674,786