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, 2022	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022	Jun. 30, 2021
Earnings Per Share [Abstract]
Net earnings available to Alleghany stockholders		$ (171,614)	$ 125,681	$ 403,651	$ 230,041	$ (45,933)	$ 633,692
Effect of dilutive securities		0		(0)		0	0
Income available to common stockholders for diluted earnings per share		$ (171,600)		$ 403,700		$ (45,900)	$ 633,700
Weighted average common shares outstanding applicable to basic earnings per share		13,455,198		13,919,489		13,488,993	13,954,449
Effect of dilutive securities		0		0		0	0
Adjusted weighted average common shares outstanding applicable to diluted earnings per share		13,455,198		13,919,489		13,488,993	13,954,449
Contingently issuable shares	[1]					54,612	59,972

[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.