STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit	Total
Period Start Balance at Dec. 31, 2014	$ 90	$ 672,739	$ 2,590	$ (463,759)	$ 211,660
Balance (in shares) at Dec. 31, 2014	89,592,779
Increase (Decrease) in Stockholders' Equity
Shares Issued under Stock Option and Employee Stock Purchase Plans	$ 1	4,310			4,311
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)	755,316
Shares Issued in Underwritten Offering	$ 8	188,832			188,840
Shares Issued in Underwritten Offering (in shares)	8,337,500
Share-Based Compensation		12,774			12,774
Foreign Currency Translation Adjustments			15		15
Unrealized Gains (Losses) on Marketable Securities			(298)		(298)
Net Loss				(127,197)	(127,197)
Period End Balance at Dec. 31, 2015	$ 99	878,655	2,307	(590,956)	290,105
Balance (in shares) at Dec. 31, 2015	98,685,595
Increase (Decrease) in Stockholders' Equity
Shares Issued under Stock Option and Employee Stock Purchase Plans	$ 1	534			535
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)	158,152
Shares Issued in Connection with Cantor Agreement	$ 3	13,943			13,946
Shares Issued in Connection with Cantor Agreement (in shares)	3,303,800
Shares Issued in Connection with the Kolltan Acquisition	$ 18	73,379			73,397
Shares Issued in Connection with the Kolltan Acquisition (in shares)	18,257,996
Shares Issued in Connection with Kolltan Severance		427			427
Shares Issued in Connection with Kolltan Severance (in shares)	111,111
Share-Based Compensation		15,317			15,317
Unrealized Gains (Losses) on Marketable Securities			234		234
Net Loss				(128,530)	(128,530)
Period End Balance at Dec. 31, 2016	$ 121	982,255	2,541	(719,486)	$ 265,431
Balance (in shares) at Dec. 31, 2016	120,516,654				120,516,654
Increase (Decrease) in Stockholders' Equity
Shares Issued under Stock Option and Employee Stock Purchase Plans		265			$ 265
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)	173,712
Shares Issued in Connection with Cantor Agreement	$ 18	51,007			51,025
Shares Issued in Connection with Cantor Agreement (in shares)	17,722,863
Shares Issued in Connection with Kolltan Severance		344			344
Shares Issued in Connection with Kolltan Severance (in shares)	107,175
Share-Based Compensation		12,312			12,312
Unrealized Gains (Losses) on Marketable Securities			23		23
Net Loss				(93,031)	(93,031)
Period End Balance at Dec. 31, 2017	$ 139	$ 1,046,183	$ 2,564	$ (812,517)	$ 236,369
Balance (in shares) at Dec. 31, 2017	138,520,404				138,520,404