Securities - Amortized Cost and Estimated Fair Value of Securities by Contractual Maturity (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale, Amortized Cost
Due in one year or less	$ 1,817,972	$ 3,398,727
Due after one year through five years	97,858,493	75,887,288
Due after five years through ten years	31,088,580	55,691,854
Due after ten years	354,597,314	381,028,427
Amortized Cost	485,362,359	516,006,296
Available-for-sale, Estimated Fair Value
Due in one year or less	1,820,005	3,421,576
Due after one year through five years	94,124,451	74,589,829
Due after five years through ten years	29,855,611	54,740,055
Due after ten years	337,952,118	372,294,917
Total Fair Value	$ 463,752,185	$ 505,046,377