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, 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 at the market agreement	$ 3	29,019			29,022
Shares issued in connection with at the market 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
Stock-based compensation		8,081			8,081
Unrealized gain on marketable securities			19		19
Net loss				(151,184)	(151,184)
Period End Balance (ASC 606) at Dec. 31, 2018				1,263	1,263
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
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 at the market agreement	$ 5	16,243			16,248
Shares issued in connection with at the market agreement (in shares)	5,011,157
Stock-based compensation		4,551			4,551
Unrealized gain 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
Increase (Decrease) in Stockholders' Equity
Shares issued under stock option and employee stock purchase plans		434			$ 434
Shares issued under stock option and employee stock purchase plans (in shares)	122,076
Shares issued in connection with at the market agreement	$ 8	29,423			29,431
Shares issued in connection with at the market agreement (in shares)	7,125,004
Shares issued in underwritten public offering	$ 15	141,346			141,361
Shares issued in underwritten public offering (in shares)	15,384,614
Stock-based compensation		3,915			3,915
Unrealized gain on marketable securities			(30)		(30)
Net loss				(59,780)	(59,780)
Period End Balance at Dec. 31, 2020	$ 40	$ 1,279,824	$ 2,589	$ (1,073,096)	$ 209,357
Balance (in shares) at Dec. 31, 2020	39,603,771				39,603,771