CONSOLIDATED 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, 2016	$ 8	$ 982,368	$ 2,541	$ (719,486)	$ 265,431
Balance (in shares) at Dec. 31, 2016	8,034,443
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)	11,581
Shares Issued in Connection with Cantor Agreement	$ 1	51,024			51,025
Shares Issued in Connection with Cantor Agreement (in shares)	1,181,524
Shares Issued in Connection with Kolltan Severance		344			344
Shares Issued in Connection with Kolltan Severance (in shares)	7,145
Share-Based Compensation		12,312			12,312
Unrealized Gains on Marketable Securities			23		23
Net Loss				(93,031)	(93,031)
Period End Balance at Dec. 31, 2017	$ 9	1,046,313	2,564	(812,517)	236,369
Balance (in shares) at Dec. 31, 2017	9,234,693
Increase (Decrease) in Stockholders' Equity
Shares Issued under Stock Option and Employee Stock Purchase Plans		419			419
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)	16,047
Shares Issued in Connection with Cantor Agreement	$ 3	29,019			29,022
Shares Issued in Connection with Cantor Agreement (in shares)	2,702,660
Shares Issued in Connection with Kolltan Severance		71			71
Shares Issued in Connection with Kolltan Severance (in shares)	4,235
Share-Based Compensation		8,081			8,081
Unrealized Gains on Marketable Securities			19		19
Adoption of ASC 606				1,263	1,263
Net Loss				(151,184)	(151,184)
Period End Balance at Dec. 31, 2018	$ 12	1,083,903	2,583	(962,438)	$ 124,060
Balance (in shares) at Dec. 31, 2018	11,957,635				11,957,635
Increase (Decrease) in Stockholders' Equity
Shares Issued under Stock Option and Employee Stock Purchase Plans		9			$ 9
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)	3,285
Shares Issued in Connection with Cantor Agreement	$ 5	16,243			16,248
Shares Issued in Connection with Cantor Agreement (in shares)	5,011,157
Share-Based Compensation		4,551			4,551
Unrealized Gains on Marketable Securities			36		36
Net Loss				(50,878)	(50,878)
Period End Balance at Dec. 31, 2019	$ 17	$ 1,104,706	$ 2,619	$ (1,013,316)	$ 94,026
Balance (in shares) at Dec. 31, 2019	16,972,077				16,972,077