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Note 1 - Significant Accounting Policies - Cumulative Effective Adopted Accounting Pronouncement (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Deferred collaboration expense	$ 8		$ 8		$ 152	$ 210
Deferred revenue	200		200		7,300	8,484
Accumulated deficit	(704,537)		(704,537)		(630,717)	$ (631,843)
Revenues	1,454	$ 8,760	17,924	$ 21,296
Research and development	22,006	17,509	61,457	50,038
Net loss	$ (29,597)	$ (15,134)	$ (73,820)	$ (46,239)
Basic and diluted net loss per common share (in dollars per share)	$ (0.28)	$ (0.18)	$ (0.73)	$ (0.58)
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	$ 19		$ 19
Deferred revenue	396		396
Accumulated deficit	378		378
Research and development	14		44
Net loss	$ 282		$ 846
Basic and diluted net loss per common share (in dollars per share)	$ 0		$ 0.01
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Deferred collaboration expense	$ 27		$ 27
Deferred revenue	596		596
Accumulated deficit	(704,159)		(704,159)
Research and development	22,021		61,501
Net loss	$ (29,315)		$ (72,974)
Basic and diluted net loss per common share (in dollars per share)	$ (0.28)		$ (0.72)
Collaborative and Other Research and Development [Member]
Revenues	$ 931	$ 6,817	$ 13,598	$ 12,543
Collaborative and Other Research and Development [Member] \| Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenues	296		888
Collaborative and Other Research and Development [Member] \| Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenues	$ 1,227		$ 14,486