Condensed Financial Information of Parent Company - Summary of Condensed Statements of Cash Flows (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities:
Net (loss) income	$ 32,760,340	$ (7,116,100)	$ (30,801,137)
Adjustments:
Other	299,262	(113,482)	(22,628)
Net adjustments	96,536,474	73,884,098	29,228,985
Net cash provided by operating activities	63,776,134	81,000,198	60,030,122
Cash flows from investing activities:
Net sale of short-term investments	(5,698,845)	(1,678,908)	4,061,475
Net cash used in investing activities	(37,606,007)	(58,443,382)	(51,318,346)
Cash Flows from Financing Activities:
Cash dividends paid	(15,658,950)	(14,822,052)	(14,085,934)
Issuance of common stock	3,249,849	15,511,457	13,822,228
Payments on lines of credit		(10,000,000)	(12,000,000)
Borrowings under lines of credit	1,000,000
Net cash used in financing activities	(11,409,101)	(9,310,595)	(12,263,706)
Net increase (decrease) in cash	14,761,026	13,246,221	(3,551,930)
Cash at beginning of year	37,833,435	24,587,214	28,139,144
Cash at end of year	52,594,461	37,833,435	24,587,214
Parent Company [Member]
Cash Flows from Operating Activities:
Net (loss) income	(32,760,000)	7,116,000	30,801,000
Adjustments:
Equity in undistributed net loss (income) of subsidiaries	42,694,000	5,343,000	(18,271,000)
Other	2,531,000	2,048,000	2,802,000
Net adjustments	45,225,000	7,391,000	(15,469,000)
Net cash provided by operating activities	12,465,000	14,507,000	15,332,000
Cash flows from investing activities:
Net sale of short-term investments		1,000	2,000
Net purchase of property and equipment	(106,000)	(788,000)	(11,000)
Investment in subsidiaries	(2,644,000)	(2,992,000)	(2,393,000)
Other	(1,000)	(1,000)
Net cash used in investing activities	(2,751,000)	(3,780,000)	(2,402,000)
Cash Flows from Financing Activities:
Cash dividends paid	(15,659,000)	(14,821,000)	(14,085,000)
Issuance of common stock	3,250,000	15,511,000	13,822,000
Payments on lines of credit		(10,000,000)	(12,000,000)
Borrowings under lines of credit	1,000,000
Net cash used in financing activities	(11,409,000)	(9,310,000)	(12,263,000)
Net increase (decrease) in cash	(1,695,000)	1,417,000	667,000
Cash at beginning of year	3,237,000	1,820,000	1,153,000
Cash at end of year	$ 1,542,000	$ 3,237,000	$ 1,820,000