Note 4 - Employee Benefit Plans - Summary of Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Exercised, number of options (in shares)	(16,007)
Employee Stock Option [Member]
Outstanding, number of options (in shares)	873,992
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 6.49
Outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 6.55
Outstanding, weighted average remaining contractual life (Year)	6 years 189 days
Outstanding, aggregate intrinsic value | $	$ 7,813,773
Exercisable, number of options (in shares)	616,615
Exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 6.14
Granted, number of options (in shares)	332,469
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 6.74
Exercised, number of options (in shares)	(16,007)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 4.91
Forfeited or expired, number of options (in shares)	(28,880)
Forfeited or expired, weighted average exercise price (in dollars per share) | $ / shares	$ 7.79
Outstanding, number of options (in shares)	1,161,574
Exercisable, weighted average remaining contractual life (Year)	4 years 7 days
Exercisable, aggregate intrinsic value | $	$ 4,399,729