Note 8 - Earnings Per Share - Calculation of Diluted Weighted-average Shares Outstanding (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Net income	$ 142,242	$ 811,728	$ 892,430	$ 952,077	$ 360,591
Less: Earnings allocated to participating securities	(3,313)		(21,681)
Net income allocated to common shareholders	$ 138,929	$ 811,728	$ 870,749	$ 952,077
Weighted-average common shares outstanding including participating securities (in shares)	3,404,065	3,438,190	3,416,900	3,438,190
Less: Average participating securities (in shares)	(75,179)		(75,083)
Less: Average unallocated ESOP Shares (in shares)	(176,102)	(181,887)	(176,102)	(181,887)
Average shares (in shares)	3,152,784	3,256,303	3,165,715	3,256,303
Basic (in dollars per share)	$ 0.04	$ 0.25	$ 0.28	$ 0.29
Net income allocated to common shareholders	$ 138,929	$ 811,728	$ 870,749	$ 952,077
Weighted-average common shares outstanding for basic earnings per common share (in shares)	3,152,784	3,256,303	3,165,715	3,256,303
Add: Dilutive effects of assumed exercises of stock options (in shares)	13,362	9,218	9,480	8,729
Weighted average shares and dilutive potential common shares (in shares)	3,166,146	3,265,521	3,175,195	3,265,032
Diluted (in dollars per share)	$ 0.04	$ 0.25	$ 0.27	$ 0.29