Reconciliation of Earnings and Share Data used in Basic and Diluted Earnings per Share Computations (Detail) - USD ($) $ in Thousands		3 Months Ended						9 Months Ended
		Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net earnings (losses) available to Alleghany stockholders		$ 126,522	$ 177,378	$ (361,218)	$ 90,432	$ 295,459	$ 440,227	$ (57,318)	$ 826,118
Effect of dilutive securities		0			0			(6,200)	0
Income available to common stockholders for diluted earnings per share		$ 126,500			$ 90,400			$ (63,500)	$ 826,100
Weighted average common shares outstanding applicable to basic earnings per share		14,275,487			14,422,581			14,309,010	14,447,794
Effect of dilutive securities		0			0			23,310	10,956
Adjusted weighted average common shares outstanding applicable to diluted earnings per share		14,275,487			14,422,581			14,332,320	14,458,750
Contingently issuable shares	[1]	0			0			32,652	50,556

[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.