Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2018	$ 177,604	$ 51	$ 733,907	$ (92)	$ (556,262)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	4,073		4,073
Exercise of stock options	13		13
Employee stock purchase plan purchases	170		170
Comprehensive loss:
Unrealized gain (loss) on investments, net	140			140
Net loss	(17,693)				(17,693)
Comprehensive loss	(17,553)
Ending Balance at Mar. 31, 2019	164,307	51	738,163	48	(573,955)
Beginning Balance at Dec. 31, 2018	177,604	51	733,907	(92)	(556,262)
Comprehensive loss:
Unrealized gain (loss) on investments, net	182
Comprehensive loss	(108,891)
Ending Balance at Sep. 30, 2019	111,948	61	777,133	89	(665,335)
Beginning Balance at Mar. 31, 2019	164,307	51	738,163	48	(573,955)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	2,367		2,367
Exercise of stock options	17		17
Comprehensive loss:
Unrealized gain (loss) on investments, net	77			77
Net loss	(17,650)				(17,650)
Comprehensive loss	(17,573)
Ending Balance at Jun. 30, 2019	149,118	51	740,547	125	(591,605)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	1,529		1,529
Exercise of stock options	13		13
Employee stock purchase plan purchases	154		154
Issuance of common stock, net of issuance costs	34,900	10	34,890
Comprehensive loss:
Unrealized gain (loss) on investments, net	(36)			(36)
Net loss	(73,730)				(73,730)
Comprehensive loss	(73,766)
Ending Balance at Sep. 30, 2019	111,948	61	777,133	89	(665,335)
Beginning Balance at Dec. 31, 2019	109,952	62	778,693	35	(668,838)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	1,326		1,326
Employee stock purchase plan purchases	229		229
Comprehensive loss:
Unrealized gain (loss) on investments, net	(46)			(46)
Net loss	(10,420)				(10,420)
Comprehensive loss	(10,466)
Ending Balance at Mar. 31, 2020	101,041	62	780,248	(11)	(679,258)
Beginning Balance at Dec. 31, 2019	109,952	62	778,693	35	(668,838)
Comprehensive loss:
Unrealized gain (loss) on investments, net	(2)
Comprehensive loss	(31,849)
Ending Balance at Sep. 30, 2020	83,169	63	783,758	33	(700,685)
Beginning Balance at Mar. 31, 2020	101,041	62	780,248	(11)	(679,258)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	1,391		1,391
Exercise of stock options	578		578
Comprehensive loss:
Unrealized gain (loss) on investments, net	141			141
Net loss	(10,016)				(10,016)
Comprehensive loss	(9,875)
Ending Balance at Jun. 30, 2020	93,135	62	782,217	130	(689,274)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	1,282		1,282
Exercise of stock options	63	1	62
Employee stock purchase plan purchases	197		197
Comprehensive loss:
Unrealized gain (loss) on investments, net	(97)			(97)
Net loss	(11,411)				(11,411)
Comprehensive loss	(11,508)
Ending Balance at Sep. 30, 2020	$ 83,169	$ 63	$ 783,758	$ 33	$ (700,685)