Stockholders' Equity (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Apr. 30, 2020	Mar. 31, 2020	Mar. 31, 2019
Common Stock
Period Start Balance	$ 83,703	$ 94,026	$ 124,060
Balance (in shares)	17,730,802	16,972,077
Shares Issued under Stock Option and Employee Stock Purchase Plans		$ 24	9
Maximum aggregate offering price of securities available for sale		18,000
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition		686	1,693
Shares Issued In Connection With Cantor Agreement		1,614	4,151
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax		(22)	19
Net Income (Loss) Available to Common Stockholders, Basic		(12,625)	(17,239)
Period End Balance		$ 83,703	112,693
Balance (in shares)		17,730,802
Net proceeds from sale of common stock		$ 1,613	4,151
Common Stock
Common Stock
Period Start Balance	$ 18	$ 17	$ 12
Balance (in shares)	17,730,802	16,972,077	11,957,635
Shares Issued under Stock Option and Employee Stock Purchase Plans (in shares)		12,573	3,507
Shares Issued In Connection With Cantor Agreement		$ 1	$ 1
Shares Issued in Connection with Cantor Agreement (in shares)		746,152	883,569
Period End Balance		$ 18	$ 13
Balance (in shares)		17,730,802	12,844,711
Aggregate gross offering price available		$ 17,900
Net proceeds from sale of common stock		1,600
Common Stock | Subsequent Event
Common Stock
Common stock issued (in shares)	1,200,000
Net proceeds from sale of common stock	$ 2,900
Additional Paid-In Capital
Common Stock
Period Start Balance	1,107,029	1,104,706	$ 1,083,903
Shares Issued under Stock Option and Employee Stock Purchase Plans		24	9
Adjustments to Additional Paid in Capital, Share-based Compensation, Requisite Service Period Recognition		686	1,693
Shares Issued In Connection With Cantor Agreement		1,613	4,150
Period End Balance		1,107,029	1,089,755
Accumulated Other Comprehensive Income.
Common Stock
Period Start Balance	2,597	2,619	2,583
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax		(22)	19
Period End Balance		2,597	2,602
Accumulated Deficit
Common Stock
Period Start Balance	$ (1,025,941)	(1,013,316)	(962,438)
Net Income (Loss) Available to Common Stockholders, Basic		(12,625)	(17,239)
Period End Balance		$ (1,025,941)	$ (979,677)