Note 9 - Stockholders' Equity - Summary of Option Activity (Details) - USD ($)		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017	[1]
Outstanding (in shares)	[1]	1,536,282
Outstanding, weighted average exercise price (in dollars per share)	[1]	$ 13.86
Outstanding, weighted average contractual term (Year)		4 years 197 days	[2]	4 years 54 days
Outstanding, aggregate intrinsic value		$ 12,214,422	[2]	$ 27,771,635
Granted (in shares)		280,423
Granted, weighted average exercise price (in dollars per share)		$ 23.56
Exercised (in shares)	[3]	(303,379)
Exercised, weighted average exercise price (in dollars per share)	[3]	$ 9.79
Exercised, aggregate intrinsic value	[3]	$ 7,196,175
Canceled / Expired (in shares)		(50,751)
Canceled / Expired, weighted average exercise price (in dollars per share)		$ 13.79
Outstanding (in shares)		1,462,575	[2]	1,536,282
Outstanding, weighted average exercise price (in dollars per share)		$ 16.41	[2]	$ 13.86
Vested and exercisable (in shares)	[4]	504,988
Vested and exercisable, weighted average exercise price (in dollars per share)	[4]	$ 11.51
Vested and exercisable, weighted average contractual term (Year)	[4]	2 years 237 days
Vested and exercisable, aggregate intrinsic value	[4]	$ 6,482,151
Expected to vest (in shares)		957,587
Expected to vest, weighted average exercise price (in dollars per share)		$ 18.99
Expected to vest, weighted average contractual term (Year)		5 years 197 days
Total (in shares)		1,462,575

[1] The aggregate intrinsic value represents the difference between the exercise price and $31.84, the closing price of our stock on December 31, 2017, for all in-the-money options outstanding.

[2] 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.

[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 $23.64, the closing price of our stock on December 31, 2018, for all in-the-money options vested and exercisable as of that date.