Note 4 - Earnings Per Share - Basic and Diluted (Details) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
Net income	$ 8,766,017	$ 9,184,965	$ 6,676,467	$ 6,373,489	$ 17,950,982	$ 13,049,956
Basic earnings (loss) per common share (in dollars per share) (in dollars per share)	$ 0.67		$ 0.54		$ 1.36	$ 1.08
Diluted earnings (loss) per common share (in dollars per share) (in dollars per share)	$ 0.65		$ 0.53		$ 1.33	$ 1.07
Weighted average common shares outstanding (in shares)	13,170,283		12,335,077		13,151,833	12,064,349
Weighted average common shares issuable upon exercise of stock options and under the employee stock purchase plan (in shares)	346,309		181,397		350,672	170,863
Weighted average common and common equivalent shares outstanding (in shares)	13,516,592		12,516,474		13,502,505	12,235,212