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Reinsurance - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Insurance [Abstract]      
Paid losses recoverable outstanding due date 90 days    
Paid losses recoverable past due $ 0    
Received reinsurance recoveries $ 1,600,000 $ 500,000 $ 1,000,000
Unsecured reinsurance recoverable by reinsurer reporting threshold 1.50%