Note 3 - Investment Securities - Securities by Contractual Maturity (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Securities held to maturity:
Securities held to maturity, due in one year or less, amortized cost	$ 1,710,335
Securities held to maturity, due in one year or less, fair value	1,711,153
Securities held to maturity, due after one year through five years, amortized cost	27,876,391
Securities held to maturity, due after one year through five years, fair value	27,826,476
Securities held to maturity, due after five years, amortized cost	365,834,469
Securities held to maturity, due after five years, fair value	359,003,651
Securities held to maturity, amortized cost	395,421,195
Securities held to maturity, fair value	388,541,280	$ 379,749,804
Securities available for sale:
Securities available for sale, due in one year or less, amortized cost	3,153,996
Securities available for sale, Due in one year or less, fair value	3,156,871
Securities available for sale, due after one year through five years, amortized cost	25,663,131
Securities available for sale, Due after one year through five years, fair value	25,220,152
Securities available for sale, due after five years, amortized cost	73,254,659
Securities available for sale, Due after five years, fair value	71,213,823
Securities available for sale, single maturity, amortized cost	102,071,786
Securities available for sale, single maturity, fair value	99,590,846
Securities available for sale, amortized cost	265,005,513
Securities available for sale, fair value	255,323,442
Callable Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	59,253,613
Securities available for sale, no single maturity, fair value	57,823,552
Securities available for sale, callable, amortized cost	59,253,613
Securities available for sale, callable, fair value	57,823,552
US States and Political Subdivisions Debt Securities [Member]
Securities held to maturity:
Securities held to maturity, fair value	387,504,760	378,699,804
US States and Political Subdivisions Debt Securities [Member] | Callable Securities [Member]
Securities held to maturity:
Securities held to maturity, callable, amortized cost	229,992,901
Securities held to maturity, callable, fair value	226,439,711
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	50,248,182
Securities available for sale, no single maturity, fair value	49,027,162
Securities available for sale, callable, amortized cost	50,248,182
Securities available for sale, callable, fair value	49,027,162
US Government-sponsored Enterprises Debt Securities [Member] | Callable Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	4,998,969
Securities available for sale, no single maturity, fair value	4,847,750
Securities available for sale, callable, amortized cost	4,998,969
Securities available for sale, callable, fair value	4,847,750
Corporate Debt Securities [Member] | Callable Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	4,006,462
Securities available for sale, no single maturity, fair value	3,948,640
Securities available for sale, callable, amortized cost	4,006,462
Securities available for sale, callable, fair value	3,948,640
Residential Mortgage Backed Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	162,933,727
Securities available for sale, no single maturity, fair value	155,732,596
Securities available for sale, callable, amortized cost	162,933,727
Securities available for sale, callable, fair value	155,732,596
Other Securities [Member]
Securities held to maturity:
Securities held to maturity, fair value	$ 1,036,520	$ 1,050,000