Investments - Summary of Gross Realized Gains and Losses from Investments before Applicable Income Taxes (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Gain (Loss) on Securities [Line Items]
Gross realized gains	$ 2,137,000	$ 1,130,000	$ 3,282,000	$ 3,679,000
Gross realized losses	620,000	33,000	2,683,000	33,000
Net realized gains	1,517,310	1,097,306	598,971	3,646,281
Fixed Maturities [Member]
Gain (Loss) on Securities [Line Items]
Gross realized gains	12,000	45,000	12,000	50,000
Gross realized losses	1,000	30,000	45,000	30,000
Equity Securities [Member]
Gain (Loss) on Securities [Line Items]
Gross realized gains	2,125,000	1,085,000	3,270,000	3,629,000
Gross realized losses	$ 619,000	$ 3,000	$ 2,638,000	$ 3,000