Note 13 - Stock-based Compensation - Common Stock Option Activity (Details) - $ / shares	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Common Stock Options Outstanding, number of shares (in shares)	1,068,017	821,470
Common Stock Options Outstanding, weighted-average exercise price per share (in dollars per share)	$ 9.78	$ 11.00
Common Stock Options Granted, number of shares (in shares)	161,479	249,283
Common Stock Options Granted, weighted-average exercise price per share (in dollars per share)	$ 5.36	$ 5.76
Common Stock Options Exercised, number of shares (in shares)	0	0
Common Stock Options Exercised, weighted-average exercise price per share (in dollars per share)	$ 0	$ 0
Common Stock Options Forfeited, number of shares (in shares)	0	(2,736)
Common Stock Options Forfeited, weighted-average exercise price per share (in dollars per share)	$ 0	$ 11.00
Common Stock Options Outstanding, number of shares (in shares)	1,229,496	1,068,017
Common Stock Options Outstanding, weighted-average exercise price per share (in dollars per share)	$ 9.20	$ 9.78
Common Stock Options Exercisable, number of shares (in shares)	901,829	818,734
Common Stock Options Exercisable, weighted-average exercise price per share (in dollars per share)	$ 10.52	$ 11.00