Stockholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Common Stock
Period Start Balance	$ 238,444	$ 83,703	$ 94,026	$ 109,626	$ 112,693	$ 124,060	$ 94,026	$ 124,060
Balance (in shares)			16,972,077				16,972,077
Shares Issued under Stock Option and Employee Stock Purchase Plans	194		$ 24			9
Shares Issued In Connection With Cantor Agreement	4,297	23,692	1,614	2,463	7,212	4,151
Shares Issued in Underwritten Offering		141,361
Share-Based Compensation	1,250	722	686	707	1,464	1,693
Unrealized Gain (Loss) on Marketable Securities	14	(3)	(22)	(3)	36	19	$ (11)	52
Net Loss	(14,224)	(11,031)	(12,625)	(11,413)	(11,779)	(17,239)	(37,880)	(40,431)
Period End Balance	$ 229,975	$ 238,444	83,703	101,380	109,626	112,693	$ 229,975	101,380
Balance (in shares)	39,562,472						39,562,472
Net proceeds from sale of common stock							$ 170,964	13,826
Underwritten Public Offering
Common Stock
Common stock issued (in shares)		15,384,614
Net proceeds from sale of common stock		$ 141,400
Common Stock
Common Stock
Period Start Balance	$ 39	$ 18	$ 17	$ 15	$ 13	$ 12	$ 17	$ 12
Balance (in shares)	39,093,868	17,730,802	16,972,077	14,816,917	12,844,711	11,957,635	16,972,077	11,957,635
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)	68,204		12,573			3,507
Shares Cancelled under Stock Option and Employee Stock Purchase Plans				(222)
Shares Issued In Connection With Cantor Agreement	$ 1	$ 6	$ 1	$ 1	$ 2	$ 1
Shares Issued in Connection with Cantor Agreement (in shares)	400,400	5,978,452	746,152	1,087,603	1,972,428	883,569	7,100,000
Shares Issued in Underwritten Offering		$ 15
Shares Issued in Underwritten Offering (in shares)		15,384,614
Period End Balance	$ 40	$ 39	$ 18	$ 16	$ 15	$ 13	$ 40	$ 16
Balance (in shares)	39,562,472	39,093,868	17,730,802	15,904,520	14,816,917	12,844,711	39,562,472	15,904,520
Net proceeds from sale of common stock							$ 29,600
Aggregate gross offering price available	$ 13,800						13,800
Additional Paid-In Capital
Common Stock
Period Start Balance	1,272,783	$ 1,107,029	$ 1,104,706	$ 1,098,429	$ 1,089,755	$ 1,083,903	1,104,706	$ 1,083,903
Shares Issued under Stock Option and Employee Stock Purchase Plans	194		24			9
Shares Issued In Connection With Cantor Agreement	4,296	23,686	1,613	2,462	7,210	4,150
Shares Issued in Underwritten Offering		141,346
Share-Based Compensation	1,250	722	686	707	1,464	1,693
Period End Balance	1,278,523	1,272,783	1,107,029	1,101,598	1,098,429	1,089,755	1,278,523	1,101,598
Accumulated Other Comprehensive Income
Common Stock
Period Start Balance	2,594	2,597	2,619	2,638	2,602	2,583	2,619	2,583
Unrealized Gain (Loss) on Marketable Securities	14	(3)	(22)	(3)	36	19
Period End Balance	2,608	2,594	2,597	2,635	2,638	2,602	2,608	2,635
Accumulated Deficit
Common Stock
Period Start Balance	(1,036,972)	(1,025,941)	(1,013,316)	(991,456)	(979,677)	(962,438)	(1,013,316)	(962,438)
Net Loss	(14,224)	(11,031)	(12,625)	(11,413)	(11,779)	(17,239)
Period End Balance	$ (1,051,196)	$ (1,036,972)	$ (1,025,941)	$ (1,002,869)	$ (991,456)	$ (979,677)	$ (1,051,196)	$ (1,002,869)