Condensed Financial Information of Parent Company - Summary of Condensed Statements of Income and Comprehensive Income (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
Revenues
Total revenues	$ 190,758,831	$ 185,716,027	$ 183,580,716	$ 178,970,963	$ 177,196,114	$ 175,311,263	$ 169,846,867	$ 166,068,776	$ 739,026,537	$ 688,423,020	$ 636,387,263
Expenses
Premium paid on purchase of treasury stock											5,780,000
Interest									1,593,437	1,657,647	1,111,441
Total expenses	130,442,303	114,386,379	128,005,914	114,433,458	113,368,960	111,174,963	103,193,915	95,578,065	726,912,075	647,094,613	608,794,995
Income (loss) before income tax benefit and equity in undistributed net (loss) income of subsidiaries									12,114,462	41,328,407	27,592,268
Income tax benefit									(4,998,362)	(10,527,270)	(6,602,235)
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
Statements of Comprehensive Income
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
Other comprehensive income (loss), net of tax
Unrealized gain (loss) - subsidiaries									3,811,151	(1,386,391)	(3,322,149)
Comprehensive income									7,161,783	27,773,122	16,410,508
Parent Company [Member]
Revenues
Dividends from subsidiaries									13,000,000	13,000,000	3,875,000
Other									2,131,000	1,759,000	2,028,000
Total revenues									15,131,000	14,759,000	5,903,000
Expenses
Operating expenses									1,433,000	1,413,000	2,451,000
Premium paid on purchase of treasury stock											5,780,000
Interest									1,929,000	1,747,000	1,066,000
Total expenses									3,362,000	3,160,000	9,297,000
Income (loss) before income tax benefit and equity in undistributed net (loss) income of subsidiaries									11,769,000	11,599,000	(3,394,000)
Income tax benefit									690,000	931,000	1,028,000
Income (loss) before equity in undistributed net (loss) income of subsidiaries									12,459,000	12,530,000	(2,366,000)
Equity in undistributed net (loss) income of subsidiaries									(5,343,000)	18,271,000	23,356,000
Net income									7,116,000	30,801,000	20,990,000
Statements of Comprehensive Income
Net income									7,116,000	30,801,000	20,990,000
Other comprehensive income (loss), net of tax
Unrealized gain (loss) - subsidiaries									46,000	(3,028,000)	(4,579,000)
Other comprehensive income (loss), net of tax									46,000	(3,028,000)	(4,579,000)
Comprehensive income									$ 7,162,000	$ 27,773,000	$ 16,411,000