Note 9 - Stockholders' Equity - Summary of Option Activity (Details) - USD ($)		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018	[1]
Outstanding (in shares)	[1]	1,462,575
Outstanding, weighted average exercise price (in dollars per share)	[1]	$ 16.41
Outstanding, weighted average contractual term (Year)		4 years 113 days	[2]	4 years 197 days
Outstanding, aggregate intrinsic value		$ 13,366,595	[2]	$ 12,214,422
Granted (in shares)		195,379
Granted, weighted average exercise price (in dollars per share)		$ 35.53
Exercised (in shares)	[3]	(509,693)
Exercised, weighted average exercise price (in dollars per share)	[3]	$ 9.53
Exercised, aggregate intrinsic value	[3]	$ 11,488,292
Canceled / Expired (in shares)		(101,167)
Canceled / Expired, weighted average exercise price (in dollars per share)		$ 17.90
Outstanding (in shares)		1,047,094	[2]	1,462,575
Outstanding, weighted average exercise price (in dollars per share)		$ 23.19	[2]	$ 16.41
Vested and exercisable (in shares)	[4]	309,501
Vested and exercisable, weighted average exercise price (in dollars per share)	[4]	$ 19.64
Vested and exercisable, weighted average contractual term (Year)	[4]	3 years 182 days
Vested and exercisable, aggregate intrinsic value	[4]	$ 5,046,733
Expected to vest (in shares)		737,593
Expected to vest, weighted average exercise price (in dollars per share)		$ 24.68
Expected to vest, weighted average contractual term (Year)		4 years 237 days
Total (in shares)		1,047,094

[1] The aggregate intrinsic value represents the difference between the exercise price and $23.64, the closing price of our stock on December 31, 2018, for all in-the-money options outstanding.

[2] The aggregate intrinsic value represents the difference between the exercise price and $35.95, the closing price of our stock on December 31, 2019, for all in-the-money options outstanding.

[3] The aggregate intrinsic value of shares exercised represents the difference between the exercise price and the closing price of our stock on the date of exercise.

[4] The aggregate intrinsic value represents the difference between the exercise price and $35.95, the closing price of our stock on December 31, 2019, for all in-the-money options vested and exercisable as of that date.