Losses and Loss Adjustment Reserve - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018 USD ($) Reinsurer	Dec. 31, 2017 USD ($)
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded rate of premiums under insurance program	100.00%
Document Period End Date	Sep. 30, 2018
Premiums written net	$ 469,065,000
Ceded unpaid loss	98,500,000
Prepaid reinsurance premiums	344,955,000	$ 321,017,000
Reinsurance Recoverable for Paid Claims and Claims Adjustments	40,900
Increase (Decrease) in Loss and Loss Adjustment Expense Reserve	2,100,000
Reserve for losses and loss adjustment expense	$ 98,500,000
Number of reinsurers | Reinsurer	2
Wright National Flood Insurance Company [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded rate of premiums under insurance program	100.00%
Expenses and allowance rate received in premiums	30.90%
Premiums written net	$ 467,700,000
Wright National Flood Insurance Company [Member] | Quota Share Agreement
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Effective cedes rate under quota share agreement	100.00%
Commission rate, percent of ceded written premiums	30.50%
Ceded amount	$ 1,400,000
Wright National Flood Insurance Company [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Loss Adjustment Expense	92,519
Case Reserves	161,758
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Amount	$ 485,000