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Losses and Loss Adjustment Reserve - Additional Information (Detail)
12 Months Ended
Dec. 31, 2018
USD ($)
Reinsurer
Dec. 31, 2017
USD ($)
Sep. 30, 2018
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, 2018    
Premiums written net $ 619,206,000 $ 604,610,000  
Reinsurance Recoverables on Unpaid Losses, Gross 65,400,000 477,800,000  
Prepaid reinsurance premiums 337,920,000 321,017,000  
Increase (Decrease) in Loss and Loss Adjustment Expense Reserve 200,000 1,100,000  
Reserve For Losses And Loss Adjustment Expenses For Reinsurance Liability $ 65,400,000 477,800,000  
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 30.00%   30.90%
Premiums written net $ 617,200,000 602,900,000  
Operating Expenses $ 2,300,000    
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 $ 2,000,000.0 $ 1,700,000  
Wright National Flood Insurance Company [Member]      
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]      
Loss Adjustment Expense 99,349    
Case Reserves 0    
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred but Not Reported (IBNR) Claims, Amount $ 100,000