Note 7 - Stock-based Compensation - Stock Plan Activities (Details) shares in Thousands	6 Months Ended
	Jun. 30, 2021 $ / shares shares
Incentive Plan [Member]
Awards available, beginning balance (in shares)	4,592
Options outstanding, beginning balance (in shares)	24,885
Weighted average exercise price, beginning balance (in dollars per share) | $ / shares	$ 6.52
Plan amendment (in shares)	7,500
Awards available, Restricted stock awards granted (in shares)	(100)
Awards available, stock option awards granted (in shares)	(1,928)
Options outstanding, stock option awards granted (in shares)	1,928
Weighted average exercise price, stock option awards granted (in dollars per share) | $ / shares	$ 11.47
Options outstanding, stock option awards exercised (in shares)	(1,619)
Weighted average exercise price, stock option awards exercised (in dollars per share) | $ / shares	$ 4.23
Awards available, stock option awards cancelled (in shares)	105
Options outstanding, stock option awards cancelled (in shares)	(105)
Weighted average exercise price, stock option awards cancelled (in dollars per share) | $ / shares	$ 7.59
Awards available, ending balance (in shares)	10,169
Options outstanding, ending balance (in shares)	25,089
Weighted average exercise price, ending balance (in dollars per share) | $ / shares	$ 7.04
Inducement Plan [Member]
Awards available, beginning balance (in shares)	229
Options outstanding, beginning balance (in shares)	4,171
Weighted average exercise price, beginning balance (in dollars per share) | $ / shares	$ 3.88
Awards available, stock option awards granted (in shares)	(138)
Options outstanding, stock option awards granted (in shares)	138
Weighted average exercise price, stock option awards granted (in dollars per share) | $ / shares	$ 8.52
Options outstanding, stock option awards exercised (in shares)	(30)
Weighted average exercise price, stock option awards exercised (in dollars per share) | $ / shares	$ 2.54
Awards available, stock option awards cancelled (in shares)	78
Options outstanding, stock option awards cancelled (in shares)	(78)
Weighted average exercise price, stock option awards cancelled (in dollars per share) | $ / shares	$ 3.57
Awards available, ending balance (in shares)	169
Options outstanding, ending balance (in shares)	4,201
Weighted average exercise price, ending balance (in dollars per share) | $ / shares	$ 4.05