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	877	0	877
Share-based compensation expense, shares	250
Sales of common stock	$ 19	13,217	0	13,236
Sales of common stock, shares	19,062
Net loss	$ 0	0	(23,199)	(23,199)
Ending balances at Sep. 30, 2019	$ 119	564,227	(569,315)	(4,969)
Ending balances, shares at Sep. 30, 2019	119,417
Beginning balances at Jun. 30, 2019	$ 110	558,334	(563,174)	(4,730)
Beginning balances, shares at Jun. 30, 2019	110,223
Share-based compensation expense	$ 0	258	0	258
Share-based compensation expense, shares	0
Sales of common stock	$ 9	5,635	0	5,644
Sales of common stock, shares	9,194
Net loss	$ 0	0	(6,141)	(6,141)
Ending balances at Sep. 30, 2019	$ 119	564,227	(569,315)	(4,969)
Ending balances, shares at Sep. 30, 2019	119,417
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	793	0	793
Share-based compensation expense, shares	201
Exercise of options	$ 0	107	0	107
Exercise of options, shares	102
Sales of common stock	$ 18	8,930	0	8,948
Sales of common stock, shares	17,800
Net loss	$ 0	0	(10,064)	(10,064)
Ending balances at Sep. 30, 2020	$ 144	578,326	(582,663)	(4,193)
Ending balances, shares at Sep. 30, 2020	143,906
Beginning balances at Jun. 30, 2020	$ 143	577,172	(579,837)	(2,522)
Beginning balances, shares at Jun. 30, 2020	143,433
Share-based compensation expense	$ 0	449	0	449
Share-based compensation expense, shares	201
Exercise of options	$ 0	104	0	104
Exercise of options, shares	98
Sales of common stock	$ 1	601	0	602
Sales of common stock, shares	375
Net loss	$ 0	0	(2,826)	(2,826)
Ending balances at Sep. 30, 2020	$ 144	$ 578,326	$ (582,663)	$ (4,193)
Ending balances, shares at Sep. 30, 2020	143,906