Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands		Dec. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Equity securities, cost		$ 2,552,722	$ 2,051,996
Debt securities, amortized cost		15,727,143	14,898,522
Allowance for credit losses	[1]	$ 502	$ 2,579
Common stock, Shares authorized		22,000,000	22,000,000
Common stock, Shares issued		17,459,961	17,459,961
Treasury stock, shares		3,861,426	3,418,781

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