Long-Term Debt - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2006	Nov. 09, 2006
Debt Instrument [Line Items]
Surplus note	$ 9,926,000	$ 11,397,000
Principal repayments of Surplus Note	1,500,000	1,500,000	$ 1,500,000
Surplus during the term of surplus note	50,000,000
Interest expense	$ 200,000	$ 300,000	$ 300,000
Surplus Note [Member]
Debt Instrument [Line Items]
Unsecured term loan agreement					$ 25,000,000.0
Debt term				20 years
Effective interest rate	2.32%	2.89%	2.47%
Principal repayments of Surplus Note	$ 368,000
Maturity year of debt instrument	through 2026
Net written premium to surplus ratio	200.00%
Gross written premium to surplus ratio	600.00%
Surplus Note [Member] | Hurricane [Member]
Debt Instrument [Line Items]
Surplus and Reinsurance sufficient to cover in excess of UPCIC probable maximum loss	1-in-100 year