Securities - Summary of Unrealized Loss Information for Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months Fair Value	$ 134,490,145	$ 111,299,318
Less than 12 months Unrealized Losses	4,887,857	1,895,666
12 months or more Fair Value	316,802,471	344,493,101
12 months or more Unrealized Losses	16,807,089	10,165,747
Total Fair Value	451,292,616	455,792,419
Total Unrealized Losses	21,694,946	12,061,413
Obligations of U.S. Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months Fair Value	15,119,779	15,681,866
Less than 12 months Unrealized Losses	647,021	223,534
12 months or more Fair Value	80,176,945	160,766,691
12 months or more Unrealized Losses	3,622,890	3,975,488
Total Fair Value	95,296,724	176,448,557
Total Unrealized Losses	4,269,911	4,199,022
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months Fair Value	93,958,426	88,499,852
Less than 12 months Unrealized Losses	3,434,432	1,613,091
12 months or more Fair Value	171,878,931	116,753,236
12 months or more Unrealized Losses	9,127,143	3,714,175
Total Fair Value	265,837,357	205,253,088
Total Unrealized Losses	12,561,575	5,327,266
State, County and Municipal [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months Fair Value	25,411,940	7,117,600
Less than 12 months Unrealized Losses	806,404	59,041
12 months or more Fair Value	64,746,595	66,973,174
12 months or more Unrealized Losses	4,057,056	2,476,084
Total Fair Value	90,158,535	74,090,774
Total Unrealized Losses	$ 4,863,460	$ 2,535,125