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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share Disclosure [Line Items]
Net earnings available to Alleghany stockholders	$ 182,478	$ 148,980	$ 307,687	$ 353,860
Effect of dilutive securities	0	0	0	0
Income available to common stockholders for diluted earnings per share	$ 182,500	$ 149,000	$ 307,700	$ 353,900
Weighted average common shares outstanding applicable to basic earnings per share	15,994,969	16,442,556	16,004,596	16,567,155
Effect of dilutive securities	8,054
Adjusted weighted average common shares outstanding applicable to diluted earnings per share	16,003,023	16,442,556	16,004,596	16,567,155