Reconciliation of Earnings and Share Data used in Basic and Diluted Earnings per Share Computations (Detail) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net earnings (losses) available to Alleghany stockholders		$ 403,651	$ 230,041	$ 177,378	$ (361,218)	$ 633,692	$ (183,840)
Effect of dilutive securities		0		0		0	(8,400)
Income available to common stockholders for diluted earnings per share		$ 403,700		$ 177,400		$ 633,700	$ (192,200)
Weighted average common shares outstanding applicable to basic earnings per share		13,919,489		14,310,874		13,954,449	14,325,772
Effect of dilutive securities		0		0		0	35,797
Adjusted weighted average common shares outstanding applicable to diluted earnings per share		13,919,489		14,310,874		13,954,449	14,361,569
Contingently issuable shares	[1]					59,972	16,758

[1] Contingently issuable shares were potentially available in the periods presented, but were not included in the diluted earnings per share computations because the impact was anti-dilutive to the earnings per share calculation.