Note 2 - Investment Securities - Securities by Contractual Maturity (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Securities held to maturity:
Securities held to maturity, due in one year or less, amortized cost	$ 1,732,495
Securities held to maturity, due in one year or less, fair value	1,737,807
Securities held to maturity, due after one year through five years, amortized cost	25,021,326
Securities held to maturity, due after one year through five years, fair value	25,051,928
Securities held to maturity, due after five years, amortized cost	373,298,523
Securities held to maturity, due after five years, fair value	370,462,633
Securities held to maturity, amortized cost	400,052,344
Securities held to maturity, fair value	397,252,368	$ 379,749,804
Securities available for sale:
Securities available for sale, due in one year or less, amortized cost	2,882,932
Securities available for sale, Due in one year or less, fair value	2,891,934
Securities available for sale, due after one year through five years, amortized cost	25,265,433
Securities available for sale, Due after one year through five years, fair value	24,969,540
Securities available for sale, due after five years, amortized cost	73,034,280
Securities available for sale, Due after five years, fair value	71,548,749
Securities available for sale, single maturity, amortized cost	101,182,645
Securities available for sale, single maturity, fair value	99,410,223
Securities available for sale, amortized cost	265,974,360
Securities available for sale, fair value	257,944,671
Callable Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	57,729,505
Securities available for sale, no single maturity, fair value	56,865,270
Securities available for sale, callable, amortized cost	57,729,505
Securities available for sale, callable, fair value	56,865,270
US States and Political Subdivisions Debt Securities [Member]
Securities held to maturity:
Securities held to maturity, fair value	396,217,718	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	232,556,893
Securities held to maturity, callable, fair value	230,900,385
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	52,730,591
Securities available for sale, no single maturity, fair value	51,992,220
Securities available for sale, callable, amortized cost	52,730,591
Securities available for sale, callable, fair value	51,992,220
US Government-sponsored Enterprises Debt Securities [Member] | Callable Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	4,998,914
Securities available for sale, no single maturity, fair value	4,873,050
Securities available for sale, callable, amortized cost	4,998,914
Securities available for sale, callable, fair value	4,873,050
Residential Mortgage Backed Securities [Member]
Securities available for sale:
Securities available for sale, no single maturity, amortized cost	164,791,715
Securities available for sale, no single maturity, fair value	158,534,448
Securities available for sale, callable, amortized cost	164,791,715
Securities available for sale, callable, fair value	158,534,448
Other Securities [Member]
Securities held to maturity:
Securities held to maturity, fair value	$ 1,034,650	$ 1,050,000