Stock-based Compensation - Categories of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Total stock-based compensation expense	$ 1,479	$ 1,630
Stock Option Plan [Member] \| Research and Development [Member]
Total stock-based compensation expense	50	668
Stock Option Plan [Member] \| General and Administrative [Member]
Total stock-based compensation expense	1,154	841
Stock Option Plan [Member] \| Sales and Marketing [Member]
Total stock-based compensation expense	$ 275	$ 121

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

Name:	us-gaap_ShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration