Significant Accounting Policies (Narrative) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) ft²	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Certificates of deposit	$ 20,000		$ 13,000
Reimbursement from National Institutes of Health research grants	$ 1,300	$ 800
Leases, discount rate	5.50%
Reduction of non-current lease liability and offsetting reduction in right-of-use assets	$ 22,500	$ 22,500
Austin, Texas [Member]
Summary Of Significant Accounting Policies [Line Items]
Office space | ft²	6,000
Lease expiration date	Dec. 31, 2020