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Allowance for Credit Losses on Available for Sale Securities (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2020
USD ($)
Financing Receivable Allowance For Credit Losses [Line Items]  
Beginning balance $ 0
Change in allowance for credit losses on available for sale securities 8,029
Charge-offs (5,400)
Recoveries 0
Ending balance 2,579 [1]
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjusted Balance  
Financing Receivable Allowance For Credit Losses [Line Items]  
Beginning balance $ 0
[1] See Note 1(r) for further information regarding Alleghany’s adoption of new credit loss accounting guidance.