Losses and Loss Adjustment Reserve - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Reinsurer	Dec. 31, 2016 USD ($)
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded rate of premiums under insurance program	100.00%
Document Period End Date	Dec. 31, 2017
Premiums written net	$ 604,610,000	$ 591,124,000
Reinsurance Recoverables on Unpaid Losses, Gross	477,800,000	78,100,000
Prepaid reinsurance premiums	321,017,000	308,661,000
Increase (Decrease) in Loss and Loss Adjustment Expense Reserve	1,100,000
Reserve For Losses And Loss Adjustment Expenses For Reinsurance Liability	$ 477,800,000	78,100,000
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%
Percentage of reinsurance recoverables allowance	30.90%
Premiums written net	$ 602,900,000	589,500,000
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%
Wright National Flood Insurance Company [Member] | Quota Share Agreement | Reinsurer Concentration Risk [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded amount	$ 1,700,000	$ 1,600,000
Wright National Flood Insurance Company [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Loss Adjustment Expense	16,606
Case Reserves	838,307
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Amount	$ 1,500,000