Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands		Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Equity securities, cost		$ 1,358,265	$ 1,625,256
Debt securities, amortized cost		13,320,808	$ 13,798,576
Allowance for credit losses	[1]	$ 31,375
Common stock, Shares authorized		22,000,000	22,000,000
Common stock, Shares issued		17,459,961	17,459,961
Treasury stock, shares		3,150,279	3,095,333

[1] See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.