Note 6 - Earnings Per Share - Basic and Diluted (Details) - USD ($)	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share
Net income	$ 8,808,835	$ 10,445,385	$ 10,549,961	$ 7,853,935	$ 8,766,017	$ 9,184,965	$ 29,804,181	$ 25,804,917
Basic earnings per common share (in dollars per share)	$ 0.56			$ 0.60			$ 2.06	$ 1.96
Diluted earnings per common share (in dollars per share)	$ 0.55			$ 0.58			$ 2.02	$ 1.91
Weighted average common shares outstanding (in shares)	15,625,123			13,151,350			14,477,783	13,151,672
Weighted average common shares issuable upon exercise of stock options and under the employee stock purchase plan** (in shares)	297,201			356,605			308,994	357,894
Weighted average common and common equivalent shares outstanding (in shares)	15,922,324			13,507,955			14,786,777	13,509,566