Investments - Summary of Net Investment Income, Consisting Primarily of Interest and Dividends (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Marketable Securities [Line Items]
Investment income	$ 27,584,155	$ 26,478,162	$ 24,558,775
Investment expenses	(4,056,851)	(3,845,432)	(3,609,077)
Net investment income	23,527,304	22,632,730	20,949,698
Short-term investments [Member]
Marketable Securities [Line Items]
Investment income	407,580	115,763	181,154
Fixed Maturities [Member]
Marketable Securities [Line Items]
Investment income	26,143,924	25,066,582	23,636,468
Equity Securities [Member]
Marketable Securities [Line Items]
Investment income	999,335	1,187,814	707,703
Other [Member]
Marketable Securities [Line Items]
Investment income	$ 33,316	$ 108,003	$ 33,450