Note 7 - Earnings Per Share - Calculation of Diluted Weighted-average Shares Outstanding (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net income	$ 811,728	$ 583,629	$ 952,077	$ 1,075,138
Weighted-average shares outstanding:
Basic (in shares)	3,256,303	3,402,828	3,256,303	3,402,828
Diluted (in shares)	3,265,521	3,406,468	3,265,032	3,403,904
Earnings per share:
Basic (in dollars per share)	$ 0.25	$ 0.17	$ 0.29	$ 0.32
Diluted (in dollars per share)	$ 0.25	$ 0.17	$ 0.29	$ 0.32