Reconciliation of Earnings and Share Data used in Basic and Diluted Earnings per Share Computations (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share Disclosure [Line Items]
Net earnings available to Alleghany stockholders	$ 204,880,000	$ 196,294,000
Effect of dilutive securities
Income available to common stockholders for diluted earnings per share	$ 204,900,000	$ 196,300,000
Weighted average common shares outstanding applicable to basic earnings per share	16,683,864	16,822,056
Effect of dilutive securities
Adjusted weighted average common shares outstanding applicable to diluted earnings per share	16,683,864	16,822,056