Investments - Summary of Gross Realized Gains and Losses from Investments before Applicable Income Taxes (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Gain (Loss) on Securities [Line Items]
Gross realized gains	$ 2,981,000	$ 600,000	$ 6,262,000	$ 4,280,000
Gross realized losses	(483,000)	39,000	2,200,000	72,000
Net realized gains	3,463,504	561,429	4,062,475	4,207,710
Fixed Maturities [Member]
Gain (Loss) on Securities [Line Items]
Gross realized gains	5,000	87,000	16,000	138,000
Gross realized losses	25,000	39,000	70,000	69,000
Equity Securities [Member]
Gain (Loss) on Securities [Line Items]
Gross realized gains	2,976,000	$ 513,000	6,246,000	4,142,000
Gross realized losses	$ (508,000)		$ 2,130,000	$ 3,000