Nature of Operations - Additional Information (Detail) - USD ($) $ in Thousands				9 Months Ended
	Jan. 18, 2021	Dec. 31, 2020	Jan. 18, 2018	Sep. 30, 2021
Nature Of Operations [Line Items]
Quota share reinsurance percentage				100.00%
Deferred policy acquisition costs, percentage of provisional ceding commission of premium				24.00%
Policy replacement date			Jun. 01, 2021
Issuance of common stock shares	100,000			100,000
Commission percentage on aggregate replacement premium	6.00%			6.00%
Percentage of each insurance subsidiary assumes				50.00%
Minimum [Member]
Nature Of Operations [Line Items]
Aggregate replacement premium	$ 80,000			$ 80,000
Maximum [Member]
Nature Of Operations [Line Items]
Total commission	$ 3,100
United [Member]
Nature Of Operations [Line Items]
Quota share reinsurance percentage		69.50%
Allowance paid for policy acquisition		$ 4,400
Deferred policy acquisition costs, percentage of provisional ceding commission of premium		25.00%
Percentage of maximum potential acquisition costs in reinsured business		31.50%