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, 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 $7.3, $4.4, and $11.0 million for 2016, 2015, and 2014, 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		456			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	$ 886	709,020	(115)	(420,807)	288,984
Balance (in shares) at Dec. 31, 2014	88,626,042
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs of $7.3, $4.4, and $11.0 million for 2016, 2015, and 2014, 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	$ 967	904,932	(433)	(578,610)	$ 326,856
Balance (in shares) at Dec. 31, 2015	96,674,786				96,674,786
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs of $7.3, $4.4, and $11.0 million for 2016, 2015, and 2014, respectively	$ 98	144,144			$ 144,242
Issuance of common stock, net of issuance costs (in shares)	9,775,000
Exercise of common stock options and warrants	$ 23	3,388			3,411
Exercise of common stock options and warrants (in shares)	2,254,384
Issuance of common stock to fund the Company's 401(k) match	$ 3	2,148			2,151
Issuance of common stock to fund the Company's 401(k) match (in shares)	341,507
Compensation expense related to issuance of stock options and restricted stock awards	$ 8	23,724			23,732
Compensation expense related to issuance of stock options and restricted stock awards (in shares)	833,627
Purchase of employee stock purchase plan shares	$ 3	2,096			2,099
Purchase of employee stock purchase plan shares (in shares)	356,823
Net loss				(167,211)	(167,211)
Accumulated other comprehensive income			15		15
Balance at Dec. 31, 2016	$ 1,102	$ 1,080,432	$ (418)	$ (745,821)	$ 335,295
Balance (in shares) at Dec. 31, 2016	110,236,127				110,236,127