Condensed Balance Sheets (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020		Dec. 31, 2019
Condensed Balance Sheet Statements Captions [Line Items]
Equity securities, cost	$ 2,051,996		$ 1,625,256
Debt securities, amortized cost	14,898,522		13,798,576
Allowance for credit losses	2,579	[1]	0
Parent Company
Condensed Balance Sheet Statements Captions [Line Items]
Equity securities, cost	587,418		1,708
Debt securities, amortized cost	47,397		$ 179,747
Allowance for credit losses	$ 57

[1] See Note 1(r) for further information regarding Alleghany’s adoption of new credit loss accounting guidance.