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Liability for Unpaid Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Insurance [Abstract]          
Estimated of ultimate losses on storm $ 754,000   $ 754,000   $ 446,700
Prior year adverse development direct losses     311,700    
Net losses after benefit of reinsurance recoveries from prior year adverse development losses $ (39) $ 113 $ 2,227 $ 1,318  
Loss reserve development on direct basis       6,900  
Loss reserve development on net basis       $ 1,300