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Note 1 - Significant Accounting Policies - Cumulative Effective Adopted Accounting Pronouncement (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Deferred collaboration expense	$ 152		$ 152	$ 210
Deferred revenue	7,300		7,300	8,484
Accumulated deficit	(656,494)		(630,717)	$ (631,843)
Revenues	3,976	$ 9,437
Research and development	18,441	16,770
Net loss	$ (25,777)	$ (14,219)
Basic and diluted net loss per common share (in dollars per share)	$ (0.26)	$ (0.19)
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	$ 44
Deferred revenue	888
Accumulated deficit	282
Research and development	14
Net loss	$ 282
Basic and diluted net loss per common share (in dollars per share)	$ 0
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Deferred collaboration expense	$ 196
Deferred revenue	8,188
Accumulated deficit	(656,212)
Research and development	18,455
Net loss	$ (25,495)
Basic and diluted net loss per common share (in dollars per share)	$ (0.26)
Collaborative and Other Research and Development [Member]
Revenues	$ 315	$ 3,116
Collaborative and Other Research and Development [Member] \| Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenues	296
Collaborative and Other Research and Development [Member] \| Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenues	$ 611