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, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share Disclosure [Line Items]
Net earnings available to Alleghany stockholders		$ 295,459	$ 440,227	$ 295,116	$ 171,575	$ 735,686	$ 466,691
Income available to common stockholders for diluted earnings per share		$ 295,500		$ 295,100		$ 735,700	$ 466,700
Weighted average common shares outstanding applicable to basic earnings per share		14,439,545		15,180,075		14,460,400	15,284,679
Effect of dilutive securities						14,960	4,915
Adjusted weighted average common shares outstanding applicable to diluted earnings per share		14,439,545		15,180,075		14,475,360	15,289,594
Contingently issuable shares	[1]					46,973	57,766

[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.