Effects of Adopting ASU Topic 606, Balance Sheet Highlights (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current liabilities [Abstract]
Deferred revenue, current portion	$ 0	$ 1,500
Total current liabilities	1,863	4,482
Deferred revenue, non-current portion	0	1,000
Total liabilities	1,863	5,622
Accumulated deficit	(198,422)	(175,516)
Total stockholders' equity	9,888	1,553	$ 11,147	$ 15,095
Pre-Topic 606 [Member]
Current liabilities [Abstract]
Deferred revenue, current portion	1,000	1,500
Total current liabilities	2,863	4,482
Deferred revenue, non-current portion		1,000
Total liabilities	2,863	5,622
Accumulated deficit	(199,422)	(175,516)
Total stockholders' equity	8,888	1,553
Post-Topic 606 [Member]
Current liabilities [Abstract]
Deferred revenue, current portion	0	0
Total current liabilities	1,863	2,982
Deferred revenue, non-current portion		0
Total liabilities	1,863	3,122
Accumulated deficit	(198,422)	(173,016)
Total stockholders' equity	9,888	4,053
Adjustment [Member] | Effects of Adopting Topic 606 [Member] | ASU Topic 606 [Member]
Current liabilities [Abstract]
Deferred revenue, current portion	(1,000)	(1,500)
Total current liabilities	(1,000)	(1,500)
Deferred revenue, non-current portion		(1,000)
Total liabilities	(1,000)	(2,500)
Accumulated deficit	1,000	2,500
Total stockholders' equity	$ 1,000	$ 2,500