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Liabilities for Losses and Loss Expenses - Additional Information (Detail) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]    
Increase (Decrease) in liability for losses and loss expenses of prior years $ (21,564) $ (10,864)
Percentage of 2010 development 3.90% 2.10%