Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2014	$ 55,279	$ 19	$ 456,920	$ 0	$ (401,660)
Beginning balance (shares) at Dec. 31, 2014		19,170
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	26,144				26,144
Issuance of common stock, net of offering costs	92,007	$ 5	92,002
Issuance of common stock, net of offering costs (shares)		5,463
Issuance of common stock upon net exercise of common stock warrants	0	$ 0	0
Issuance of common stock upon net exercise of common stock warrants (shares)		17
Issuance of common stock upon exercise of stock options	1,441	$ 1	1,440
Issuance of common stock upon exercise of stock options (shares)		96
Issuance of common stock under Employee Stock Purchase Plan	203	$ 0	203
Issuance of common stock under Employee Stock Purchase Plan (shares)		26
Stock-based compensation	7,686		7,686
Unrealized loss on marketable securities	5,485			5,485
Reclassification of other-than-temporary loss on marketable securities included in net income (loss)	(5,485)			(5,485)
Ending balance at Dec. 31, 2015	182,760	$ 25	558,251	0	(375,516)
Ending balance (shares) at Dec. 31, 2015		24,772
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(69,707)				(69,707)
Issuance of common stock upon exercise of stock options	47	$ 0	47
Issuance of common stock upon exercise of stock options (shares)		6
Issuance of common stock under Employee Stock Purchase Plan	303	$ 0	303
Issuance of common stock under Employee Stock Purchase Plan (shares)		35
Stock-based compensation	7,353		7,353
Reclassification of other-than-temporary loss on marketable securities included in net income (loss)	0
Ending balance at Dec. 31, 2016	120,756	$ 25	565,954	0	(445,223)
Ending balance (shares) at Dec. 31, 2016		24,813
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(126,817)				(126,817)
Issuance of common stock, net of offering costs	290,591	$ 9	290,582
Issuance of common stock, net of offering costs (shares)		9,256
Issuance of common stock upon net exercise of common stock warrants	0	$ 0	0
Issuance of common stock upon net exercise of common stock warrants (shares)		26
Issuance of common stock upon exercise of stock options	$ 10,478	$ 1	10,477
Issuance of common stock upon exercise of stock options (shares)	677	677
Issuance of common stock under Employee Stock Purchase Plan	$ 358	$ 0	358
Issuance of common stock under Employee Stock Purchase Plan (shares)		36
Stock-based compensation	6,155		6,155
Reclassification of other-than-temporary loss on marketable securities included in net income (loss)	0
Ending balance at Dec. 31, 2017	$ 301,521	$ 35	$ 873,526	$ 0	$ (572,040)
Ending balance (shares) at Dec. 31, 2017		34,808