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Note 1 - Significant Accounting Policies - Cumulative Effective Adopted Accounting Pronouncement (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Deferred collaboration expense	$ 12		$ 12		$ 152	$ 210
Deferred revenue	300		300		7,300	8,484
Accumulated deficit	(674,940)		(674,940)		(630,717)	$ (631,843)
Revenues	12,494	$ 3,099	16,470	$ 12,536
Research and development	21,010	15,759	39,451	32,529
Net loss	$ (18,446)	$ (16,886)	$ (44,223)	$ (31,105)
Basic and diluted net loss per common share (in dollars per share)	$ (0.19)	$ (0.21)	$ (0.45)	$ (0.40)
Accounting Standards Update 2014-09 [Member]
Deferred collaboration expense					(58)
Deferred revenue					(1,184)
Accumulated deficit					$ 1,126
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Deferred collaboration expense	$ 29		$ 29
Deferred revenue	592		592
Accumulated deficit	563		563
Research and development	14		29
Net loss	$ 282		$ 563
Basic and diluted net loss per common share (in dollars per share)	$ 0.01		$ 0.01
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Deferred collaboration expense	$ 41		$ 41
Deferred revenue	892		892
Accumulated deficit	(674,377)		(674,377)
Research and development	21,024		39,480
Net loss	$ (18,164)		$ (43,660)
Basic and diluted net loss per common share (in dollars per share)	$ (0.18)		$ (0.44)
Collaborative and Other Research and Development [Member]
Revenues	$ 12,352	$ 2,610	$ 12,667	$ 5,726
Collaborative and Other Research and Development [Member] \| Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenues	296		592
Collaborative and Other Research and Development [Member] \| Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenues	$ 12,648		$ 13,259