Note 8 - Operating Leases (Details Textual) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Jul. 31, 2019	Dec. 31, 2018
Lessee, Operating Lease, Liability, Payments, Due, Total	$ 3,925	$ 3,925
Operating Lease, Liability, Total	3,532	3,532
Operating Lease, Right-of-Use Asset	$ 3,214	$ 3,214
Operating Lease, Weighted Average Discount Rate, Percent	4.00%	4.00%
Operating Lease, Weighted Average Remaining Lease Term	4 years 73 days	4 years 73 days
Operating Lease, Expense	$ 300	$ 800
Laboratory Facility in Culver City, CA [Member]
Lessee, Operating Lease, Term of Contract			5 years
Lessee, Operating Lease, Liability, Payments, Due, Total			$ 2,800
Operating Lease, Liability, Total			$ 2,500