Note 7 - Stock-based Compensation - Stock Plan Activities (Details) shares in Thousands	3 Months Ended
	Mar. 31, 2021 $ / shares shares
Options outstanding, stock option awards exercised (in shares)	(593)
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
Awards available, Restricted stock awards granted (in shares)	(100)
Awards available, stock option awards granted (in shares)	(1,074)
Options outstanding, stock option awards granted (in shares)	1,074
Weighted average exercise price, stock option awards granted (in dollars per share) | $ / shares	$ 10.06
Options outstanding, stock option awards exercised (in shares)	(586)
Weighted average exercise price, stock option awards exercised (in dollars per share) | $ / shares	$ 3.97
Awards available, stock option awards cancelled (in shares)	39
Options outstanding, stock option awards cancelled (in shares)	(39)
Weighted average exercise price, stock option awards cancelled (in dollars per share) | $ / shares	$ 8.15
Awards available, ending balance (in shares)	3,457
Options outstanding, ending balance (in shares)	25,334
Weighted average exercise price, ending balance (in dollars per share) | $ / shares	$ 6.72
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)	(7)
Weighted average exercise price, stock option awards exercised (in dollars per share) | $ / shares	$ 3.63
Awards available, stock option awards cancelled (in shares)	27
Options outstanding, stock option awards cancelled (in shares)	(27)
Weighted average exercise price, stock option awards cancelled (in dollars per share) | $ / shares	$ 2.44
Awards available, ending balance (in shares)	118
Options outstanding, ending balance (in shares)	4,275
Weighted average exercise price, ending balance (in dollars per share) | $ / shares	$ 4.04