Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020		Dec. 31, 2019
Statement Of Financial Position [Abstract]
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
Common stock, Shares authorized	22,000,000		22,000,000
Common stock, Shares issued	17,459,961		17,459,961
Treasury stock, shares	3,418,781		3,095,333

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