Note 8 - Stock-based Compensation - Stock Plan Activities (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Incentive Plan [Member]
Awards available, beginning balance (in shares)	968	805	468
Options outstanding, beginning balance (in shares)	21,050	17,491	14,452
Weighted average exercise price, beginning balance (in dollars per share)			$ 6.06
Awards available, plan amendment (in shares)	8,000	4,000	4,400
Awards available, Restricted stock awards granted (in shares)	(31)	(27)	(13)
Awards available, stock option awards granted (in shares)	(7,469)	(4,511)	(4,272)
Options outstanding, stock option awards granted (in shares)	7,469	4,511	4,272
Weighted average exercise price, stock option awards granted (in dollars per share)	$ 8.06	$ 3.91	$ 7.15
Options outstanding, stock option awards exercised (in shares)	(510)	(251)	(1,011)
Weighted average exercise price, stock option awards exercised (in dollars per share)	$ 3.56	$ 3.75	$ 2.92
Awards available, stock option awards cancelled (in shares)	3,124	701	222
Options outstanding, stock option awards cancelled (in shares)	(3,124)	(701)	(222)
Weighted average exercise price, stock option awards cancelled (in dollars per share)	$ 6.93	$ 6.82	$ 7.44
Awards available, ending balance (in shares)	4,592	968	805
Options outstanding, ending balance (in shares)	24,885	21,050	17,491
Weighted average exercise price, ending balance (in dollars per share)	$ 6.52	$ 5.96	$ 6.49
Inducement Plan [Member]
Awards available, beginning balance (in shares)	171
Options outstanding, beginning balance (in shares)	1,329
Weighted average exercise price, beginning balance (in dollars per share)	$ 3.60
Awards available, plan amendment (in shares)	2,900
Awards available, stock option awards granted (in shares)	(3,002)
Options outstanding, stock option awards granted (in shares)	3,002
Weighted average exercise price, stock option awards granted (in dollars per share)	$ 4.02
Awards available, stock option awards cancelled (in shares)	160
Options outstanding, stock option awards cancelled (in shares)	(160)
Weighted average exercise price, stock option awards cancelled (in dollars per share)	$ 4.15
Awards available, ending balance (in shares)	229	171
Options outstanding, ending balance (in shares)	4,171	1,329
Weighted average exercise price, ending balance (in dollars per share)	$ 3.88	$ 3.60