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Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
Claim
Jun. 30, 2017
USD ($)
Mar. 31, 2018
USD ($)
Insurance [Abstract]          
Incurred (recovered) related to prior years , net reserve developmenet $ 2,310 $ 1,109 $ 2,266 $ 1,205  
Loss reserve development on direct basis 5,300 6,600 5,300 6,600  
Loss reserve development on net basis 2,600 $ 1,100 $ 2,600 $ 1,100  
Number Of Remaining Claims | Claim     78    
Estimated of ultimate losses on storm 603,500   $ 603,500   $ 447,400
Prior year development gross losses 156,100        
Net retention benefit from prior year development losses $ 300