Note 5 - Earnings Per Share - Basic and Diluted (Details) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share
Net income	$ 10,445,385	$ 10,549,961	$ 8,766,017	$ 9,184,965	$ 20,995,346	$ 17,950,982
Basic earnings per common share (in dollars per share)	$ 0.75		$ 0.67		$ 1.51	$ 1.36
Diluted earnings per common share (in dollars per share)	$ 0.73		$ 0.65		$ 1.48	$ 1.33
Weighted average common shares outstanding (in shares)	13,919,565		13,170,283		13,904,113	13,151,833
Weighted average common shares issuable upon exercise of stock options and under the employee stock purchase plan** (in shares)	312,858		346,309		314,890	350,672
Weighted average common and common equivalent shares outstanding (in shares)	14,232,423		13,516,592		14,219,003	13,502,505