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Shareholder Equity - Stock-Based Compensation Expense (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 355,000	$ 285,000	$ 682,000	$ 686,000
Unrecognized compensation expense	0		0
2014 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense	3,400,000		$ 3,400,000
Period for recognition			2 years 11 months 23 days
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	86,000	123,000	$ 139,000	288,000
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	269,000	162,000	543,000	398,000
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	355,000	283,000	680,000	659,000
Restricted stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	0	0	0	22,000
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 0	$ 2,000	$ 2,000	$ 5,000