Accounts Payable and Accrued Expenses (Schedule of Accounts Payable and Accrued Expenses) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Accounts Payable and Accrued Expenses
Accounts payable	$ 2,935	$ 3,518
Research and development	1,206	2,862
Accrued compensation	1,371	2,009
Other	254	358
Total accounts payable and accrued expenses	$ 5,766	$ 8,747