Equity Incentive Plans (Allocation of Stock-Based Compensation Expense) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 907,000	$ 906,000	$ 1,749,000	$ 1,950,000
Research and development expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	399,000	299,000	752,000	636,000
General and administrative expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 508,000	$ 607,000	$ 997,000	$ 1,314,000