Note 9 - Short-term Borrowings - Overnight Repurchase Agreements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Repurchase Agreements with Customers [Member]
Average daily balance during the period	$ 7,475,824	$ 30,082,866
Average daily interest rate during the period	0.08%	0.07%
Maximum month-end balance during the period	$ 11,829,201	$ 59,833,229
Weighted average rate as of end of period	0.15%	0.02%
Federal Funds Purchased [Member]
Average daily balance during the period	$ 13,486,239	$ 19,105,595
Average daily interest rate during the period	1.31%	0.56%
Maximum month-end balance during the period	$ 33,650,000	$ 51,750,000
Weighted average rate as of end of period	1.24%	0.70%