Reconciliation of Earnings and Share Data used in Basic and Diluted Earnings per Share Computations (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015	[1]	Sep. 30, 2015	[1]	Jun. 30, 2015	[1]	Mar. 31, 2015	[1]	Dec. 31, 2014	[1]	Sep. 30, 2014	[1]	Jun. 30, 2014	[1]	Mar. 31, 2014	[1]	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share Disclosure [Line Items]
Net earnings available to Alleghany stockholders	$ 156,100		$ 96,500		$ 182,500		$ 125,200		$ 139,100		$ 186,300		$ 149,000		$ 204,900		$ 560,315	$ 679,239	$ 628,421
Adjustment related to redeemable noncontrolling interests																	(2,600)
Income available to common stockholders for basic earnings per share																	557,700	679,200	628,400
Effect of dilutive securities																	100
Income available to common stockholders for diluted earnings per share																	$ 557,800	$ 679,200	$ 628,400
Weighted average common shares outstanding applicable to basic earnings per share																	15,871,055	16,405,388	16,786,608
Effect of dilutive securities																	8,046
Adjusted weighted average common shares outstanding applicable to diluted earnings per share																	15,879,101	16,405,388	16,786,608

[1] Attributable to Alleghany stockholders.