Summary of Rating of Debt securities (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020		Dec. 31, 2019
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 13,337,754	[1]	$ 14,211,745
BB / Ba
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	109,300
B
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	187,500
CCC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	26,200
CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	400
Below CC
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	2,100
Not rated securities
Financing Receivable Allowance For Credit Losses [Line Items]
Debt securities	$ 146,000

[1] Includes allowance for credit losses as of March 31, 2020.  See Note 3(f) of this Form 10-Q for further information on Alleghany’s allowance for credit losses for AFS securities.