Note 3 - Investment Securities - Securities by Contractual Maturity (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Securities held to maturity:
Securities held to maturity, due in one year or less, amortized cost	$ 2,912,158
Securities held to maturity, due in one year or less, fair value	2,909,816
Securities held to maturity, due after one year through five years, amortized cost	20,838,734
Securities held to maturity, due after one year through five years, fair value	20,896,388
Securities held to maturity, due after five years, amortized cost	355,723,313
Securities held to maturity, due after five years, fair value	355,943,600
Securities held to maturity, amortized cost	379,474,205
Securities held to maturity, fair value	379,749,804	$ 320,414,899
Securities available for sale:
Securities available for sale, due in one year or less, amortized cost	1,991,755
Securities available for sale, Due in one year or less, fair value	1,996,584
Securities available for sale, due after one year through five years, amortized cost	26,984,843
Securities available for sale, Due after one year through five years, fair value	26,986,584
Securities available for sale, due after five years, amortized cost	75,608,923
Securities available for sale, Due after five years, fair value	75,738,862
Securities available for sale, single maturity, amortized cost	104,585,521
Securities available for sale, single maturity, fair value	104,722,030
Securities available for sale, amortized cost	274,058,995
Securities available for sale, fair value	272,907,907
Callable Securities [Member]
Securities available for sale:
Securities available for sale, amortized cost	62,603,036
Securities available for sale, fair value	62,805,571
US States and Political Subdivisions Debt Securities [Member]
Securities held to maturity:
Securities held to maturity, fair value	378,699,804	319,364,899
US States and Political Subdivisions Debt Securities [Member] | Callable Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	57,554,280
Securities available for sale, no single maturity, fair value	57,817,498
Securities available for sale, callable, amortized cost	57,554,280
Securities available for sale, callable, fair value	57,817,498
US Government-sponsored Enterprises Debt Securities [Member] | Callable Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	5,048,756
Securities available for sale, no single maturity, fair value	4,988,073
Securities available for sale, callable, amortized cost	5,048,756
Securities available for sale, callable, fair value	4,988,073
Residential Mortgage Backed Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	165,459,470
Securities available for sale, no single maturity, fair value	163,301,304
Securities available for sale, callable, amortized cost	165,459,470
Securities available for sale, callable, fair value	163,301,304
Other Securities [Member]
Securities held to maturity:
Securities held to maturity, fair value	1,050,000	$ 1,050,000
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	4,014,004
Securities available for sale, no single maturity, fair value	4,884,573
Securities available for sale, callable, amortized cost	4,014,004
Securities available for sale, callable, fair value	4,884,573
Callable Securities [Member] | US States and Political Subdivisions Debt Securities [Member]
Securities held to maturity:
Securities held to maturity, callable, amortized cost	208,103,672
Securities held to maturity, callable, fair value	$ 208,961,233