Condensed Statements of Shareholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Common stock	Additional paid-in capital	Accumulated deficit	Total
Beginning balances at Dec. 31, 2018	$ 100	$ 550,133	$ (546,116)	$ 4,117
Beginning balances, shares at Dec. 31, 2018	100,105
Share-based compensation expense	$ 0	619	0	619
Share-based compensation expense, shares	250
Sales of common stock	$ 10	7,582	0	7,592
Sales of common stock, shares	9,868
Net loss	$ 0	0	(17,058)	(17,058)
Ending balances at Jun. 30, 2019	$ 110	558,334	(563,171)	(4,730)
Ending balances, shares at Jun. 30, 2019	110,223
Beginning balances at Mar. 31, 2019	$ 102	551,650	(554,184)	(2,432)
Beginning balances, shares at Mar. 31, 2019	102,105
Share-based compensation expense	$ 0	269	0	269
Share-based compensation expense, shares	250
Sales of common stock	$ 8	6,415	0	6,423
Sales of common stock, shares	7,868
Net loss	$ 0	0	(8,990)	(8,990)
Ending balances at Jun. 30, 2019	$ 110	558,334	(563,171)	(4,730)
Ending balances, shares at Jun. 30, 2019	110,223
Beginning balances at Dec. 31, 2019	$ 126	568,496	(572,599)	(3,977)
Beginning balances, shares at Dec. 31, 2019	125,803
Share-based compensation expense	$ 0	344	0	344
Share-based compensation expense, shares	201
Exercise of options	$ 0	3	0	3
Exercise of options, shares	4
Sales of common stock	$ 17	8,329	0	8,346
Sales of common stock, shares	17,425
Net loss	$ 0	0	(7,238)	(7,238)
Ending balances at Jun. 30, 2020	$ 143	577,172	(579,837)	(2,522)
Ending balances, shares at Jun. 30, 2020	143,433
Beginning balances at Mar. 31, 2020	$ 131	570,128	(577,533)	(7,274)
Beginning balances, shares at Mar. 31, 2020	130,878
Share-based compensation expense	$ 0	187	0	187
Share-based compensation expense, shares	201
Exercise of options	$ 0	3	0	3
Exercise of options, shares	4
Sales of common stock	$ 12	6,854	0	6,866
Sales of common stock, shares	12,350
Net loss	$ 0	0	(2,304)	(2,304)
Ending balances at Jun. 30, 2020	$ 143	$ 577,172	$ (579,837)	$ (2,522)
Ending balances, shares at Jun. 30, 2020	143,433