Note 12 - Stock-based Compensation - Summary of Option Activity Under the Plans (Details) - $ / shares	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Outstanding, shares (in shares)	6,940,000
Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 6.35
Outstanding, Weighted Average Remaining Contractual Life (Year)	7 years 10 days		6 years 32 days
Granted, shares (in shares)	1,233,000
Granted, Weighted Average Exercise Price (in dollars per share)	$ 4.77
Cancelled, shares (in shares)	(1,486,000)
Cancelled, Weighted Average Exercise Price (in dollars per share)	$ 6.54
Exercised, shares (in shares)	(36,000)	0
Exercised, Weighted Average Exercise Price (in dollars per share)	$ 4.52
Outstanding, shares (in shares)	6,651,000		6,940,000
Outstanding, Weighted Average Exercise Price (in dollars per share)	$ 6.03		$ 6.35
Exercisable, shares (in shares)	4,136,000
Exercisable, Weighted Average Exercise Price (in dollars per share)	$ 6.61
Exercisable, Weighted Average Remaining Contractual Life (Year)	5 years 237 days
Vested and expected to vest, shares (in shares)	5,822,000
Vested and expected to vest, Weighted Average Exercise Price (in dollars per share)	$ 6.17
Vested and expected to vest, Weighted Average Remaining Contractual Life (Year)	6 years 251 days