Stockholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Common Stock
Period Start Balance	$ 83,703	$ 94,026	$ 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		$ 24		9
Shares Issued In Connection With Cantor Agreement	23,692	1,614	7,212	4,151
Shares Issued in Underwritten Offering	141,361
Share-Based Compensation	722	686	1,464	1,693
Unrealized (Loss) Gain on Marketable Securities	(3)	(22)	36	19	$ (25)	55
Net Loss	(11,031)	(12,625)	(11,779)	(17,239)	(23,656)	(29,018)
Period End Balance	$ 238,444	83,703	109,626	112,693	$ 238,444	109,626
Balance (in shares)	39,093,868				39,093,868
Net proceeds from sale of common stock					$ 166,667	11,363
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	$ 18	$ 17	$ 13	$ 12	$ 17	$ 12
Balance (in shares)	17,730,802	16,972,077	12,844,711	11,957,635	16,972,077	11,957,635
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)		12,573		3,507
Shares Cancelled under Stock Option and employee Stock Purchase Plans			(222)
Shares Issued In Connection With Cantor Agreement	$ 6	$ 1	$ 2	$ 1
Shares Issued in Connection with Cantor Agreement (in shares)	5,978,452	746,152	1,972,428	883,569	6,700,000
Shares Issued in Underwritten Offering	$ 15
Shares Issued in Underwritten Offering (in shares)	15,384,614
Period End Balance	$ 39	$ 18	$ 15	$ 13	$ 39	$ 15
Balance (in shares)	39,093,868	17,730,802	14,816,917	12,844,711	39,093,868	14,816,917
Net proceeds from sale of common stock					$ 25,300
Aggregate gross offering price available	$ 18,300				18,300
Additional Paid-In Capital
Common Stock
Period Start Balance	1,107,029	$ 1,104,706	$ 1,089,755	$ 1,083,903	1,104,706	$ 1,083,903
Shares Issued under Stock Option and Employee Stock Purchase Plans		24		9
Shares Issued In Connection With Cantor Agreement	23,686	1,613	7,210	4,150
Shares Issued in Underwritten Offering	141,346
Share-Based Compensation	722	686	1,464	1,693
Period End Balance	1,272,783	1,107,029	1,098,429	1,089,755	1,272,783	1,098,429
Accumulated Other Comprehensive Income.
Common Stock
Period Start Balance	2,597	2,619	2,602	2,583	2,619	2,583
Unrealized (Loss) Gain on Marketable Securities	(3)	(22)	36	19
Period End Balance	2,594	2,597	2,638	2,602	2,594	2,638
Accumulated Deficit
Common Stock
Period Start Balance	(1,025,941)	(1,013,316)	(979,677)	(962,438)	(1,013,316)	(962,438)
Net Loss	(11,031)	(12,625)	(11,779)	(17,239)
Period End Balance	$ (1,036,972)	$ (1,025,941)	$ (991,456)	$ (979,677)	$ (1,036,972)	$ (991,456)