Condensed Financial Information of Parent Company - Summary of Condensed Statements of Cash Flows (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities:
Net income	$ (2,778,644)	$ 7,108,574	$ (2,318,648)	$ 5,104,818	$ 5,554,166	$ 4,813,404	$ 8,584,654	$ 11,848,913	$ 7,116,100	$ 30,801,137	$ 20,990,033
Adjustments:
Other									(113,482)	(22,628)	(2,235,228)
Net adjustments									73,884,098	29,228,985	47,197,840
Net cash provided (used) by operating activities									81,000,198	60,030,122	68,187,873
Cash flows from investing activities:
Net sale of short-term investments									(1,678,908)	4,061,475	6,861,166
Net cash used in investing activities									(58,443,382)	(51,318,346)	(76,064,266)
Cash Flows from Financing Activities:
Cash dividends paid									(14,822,052)	(14,085,934)	(14,455,167)
Issuance of common stock									15,511,457	13,822,228	15,516,870
Payments on line of credit									(10,000,000)	(12,000,000)	(9,500,000)
Borrowings under lines of credit											37,000,000
Purchase of treasury stock											(28,124,675)
Net cash (used in) provided by financing activities									(9,310,595)	(12,263,706)	437,028
Net increase (decrease) in cash									13,246,221	(3,551,930)	(7,439,365)
Cash at beginning of year				24,587,214				28,139,144	24,587,214	28,139,144	35,578,509
Cash at end of year	37,833,435				24,587,214				37,833,435	24,587,214	28,139,144
Parent Company [Member]
Cash Flows from Operating Activities:
Net income									7,116,000	30,801,000	20,990,000
Adjustments:
Equity in undistributed net (loss) income of subsidiaries									5,343,000	(18,271,000)	(23,356,000)
Other									(401,000)	(27,000)	539,000
Net adjustments									4,942,000	(18,298,000)	(22,817,000)
Net cash provided (used) by operating activities									12,058,000	12,503,000	(1,827,000)
Cash flows from investing activities:
Net sale of short-term investments									1,000	2,000	498,000
Net purchase of property and equipment									(788,000)	(11,000)	(23,000)
Investment in subsidiaries									(2,992,000)	(2,393,000)	(2,427,000)
Other									(1,000)
Net cash used in investing activities									(3,780,000)	(2,402,000)	(1,952,000)
Cash Flows from Financing Activities:
Cash dividends paid									(14,821,000)	(14,085,000)	(14,454,000)
Issuance of common stock									17,960,000	16,651,000	18,468,000
Payments on line of credit									(10,000,000)	(12,000,000)	(9,500,000)
Borrowings under lines of credit											37,000,000
Purchase of treasury stock											(28,125,000)
Net cash (used in) provided by financing activities									(6,861,000)	(9,434,000)	3,389,000
Net increase (decrease) in cash									1,417,000	667,000	(390,000)
Cash at beginning of year				$ 1,820,000				$ 1,153,000	1,820,000	1,153,000	1,543,000
Cash at end of year	$ 3,237,000				$ 1,820,000				$ 3,237,000	$ 1,820,000	$ 1,153,000