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Losses and Loss Adjustment Expenses - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Reserves For Losses And Loss Adjustment Expenses [Line Items]        
Unpaid claims and claim adjustment expenses overall development $ 10,894 $ 4,626 $ 18,715 $ 10,558
2015 and 2016 Loss Years [Member        
Reserves For Losses And Loss Adjustment Expenses [Line Items]        
Unpaid claims and claim adjustment expenses overall development 9,442   17,438  
2017 Loss Year [Member] | Hurricane Irma [Member]        
Reserves For Losses And Loss Adjustment Expenses [Line Items]        
Preliminary losses estimated 267,000   267,000  
Preliminary net loss estimated $ 54,000   $ 54,000