Note 4 - Earnings Per Share - Basic and Diluted (Details) - USD ($)	3 Months Ended						9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income	$ 7,853,935	$ 8,766,017	$ 9,184,965	$ 6,107,501	$ 6,676,467	$ 6,373,489	$ 25,804,917	$ 19,157,457
Basic EPS (in dollars per share)	$ 0.60			$ 0.47			$ 1.96	$ 1.55
Diluted EPS (in dollars per share)	$ 0.58			$ 0.46			$ 1.91	$ 1.52
Weighted average common shares outstanding (in shares)	13,151,350			13,066,777			13,151,672	12,398,491
Weighted average common shares issuable upon exercise of stock options and under the employee stock purchase plan (in shares)	356,605			202,926			357,894	181,551
Weighted average common and common equivalent shares outstanding (in shares)	13,507,955			13,269,703			13,509,566	12,580,042