Condensed Financial Information of Parent Company - Summary of Condensed Balance Sheets (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Short-term investments	$ 16,748,760	$ 11,049,915
Cash	52,594,461	37,833,435	$ 24,587,214	$ 28,139,144
Property and equipment	4,690,704	7,280,415
Other	2,419,566	1,483,769
Total assets	1,832,078,267	1,737,919,778
Liabilities
Cash dividends declared to stockholders	3,948,484	3,841,820
Borrowings under lines of credit	60,000,000	59,000,000
Other	1,863,363	1,789,283
Total liabilities	1,433,208,366	1,289,223,674
Stockholders' equity	398,869,901	448,696,104	438,615,320	408,388,568
Total liabilities and stockholders' equity	1,832,078,267	1,737,919,778
Parent Company [Member]
Assets
Investment in subsidiaries/affiliates (equity method)	465,030,000	509,011,000
Short-term investments	29,000	29,000
Cash	1,542,000	3,237,000	$ 1,820,000	$ 1,153,000
Property and equipment	928,000	994,000
Other		1,479,000
Total assets	467,529,000	514,750,000
Liabilities
Cash dividends declared to stockholders	3,948,000	3,842,000
Borrowings under lines of credit	60,000,000	59,000,000
Other	4,711,000	3,212,000
Total liabilities	68,659,000	66,054,000
Stockholders' equity	398,870,000	448,696,000
Total liabilities and stockholders' equity	$ 467,529,000	$ 514,750,000