Note 4 - Earnings Per Share - Basic and Diluted (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net income	$ 10,549,961	$ 9,184,965
Basic EPS (in dollars per share)	$ 0.76	$ 0.70
Diluted EPS (in dollars per share)	$ 0.74	$ 0.68
Weighted average common shares outstanding (in shares)	13,888,661	13,133,382
Weighted average common shares issuable upon exercise of stock options and under the employee stock purchase plan (in shares)	316,923	355,035
Weighted average common and common equivalent shares outstanding (in shares)	14,205,584	13,488,417