Note 6 - Commitments and Contingencies (Details Textual)	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2015 a
Operating Lease, Expense	$ 500,000	$ 500,000	$ 1,000,000.0	$ 1,000,000.0
Headquarters and Laboratory Facilities in South San Francisco, California [Member]
Area of Real Estate Property (Acre) | a						59,000
Headquarters and Laboratory Facilities in South San Francisco, California [Member] | Other Noncurrent Assets [Member]
Security Deposit	603,000		603,000		$ 603,000
Laboratory and Office Space in Austin, Texas [Member] | Other Noncurrent Assets [Member]
Security Deposit	$ 139,000		$ 139,000		$ 139,000