Consolidated 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, 2017	$ 79	$ 528,873	$ (524,086)	$ 4,866
Beginning balances, shares at Dec. 31, 2017	78,597
Adoption of New Accounting Pronouncement, Revenue from Contracts with Customers	$ 0	0	5,220	5,220
Share-based compensation expense	$ 0	543	0	543
Share-based compensation expense, shares	76
Sales of common stock	$ 14	16,562	0	16,576
Sales of common stock, shares	14,400
Net loss	$ 0	0	(15,591)	(15,591)
Ending balances at Jun. 30, 2018	$ 93	545,978	(534,457)	11,614
Ending balances, shares at Jun. 30, 2018	93,073
Beginning balances at Mar. 31, 2018	$ 79	529,205	(525,998)	3,286
Beginning balances, shares at Mar. 31, 2018	78,613
Share-based compensation expense	$ 0	211	0	211
Share-based compensation expense, shares	60
Sales of common stock	$ 14	16,562	0	16,576
Sales of common stock, shares	14,400
Net loss	$ 0	0	(8,459)	(8,459)
Ending balances at Jun. 30, 2018	$ 93	545,978	(534,457)	11,614
Ending balances, shares at Jun. 30, 2018	93,073
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