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, 2016	[1]	Sep. 30, 2016	[1]	Jun. 30, 2016	[1]	Mar. 31, 2016	[1]	Dec. 31, 2015	[1]	Sep. 30, 2015	[1]	Jun. 30, 2015	[1]	Mar. 31, 2015	[1]	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share Disclosure [Line Items]
Net earnings available to Alleghany stockholders	$ 69,500		$ 155,800		$ 77,100		$ 154,500		$ 156,100		$ 96,500		$ 182,500		$ 125,200		$ 456,921	$ 560,315	$ 679,239
Adjustment related to redeemable noncontrolling interests																		(2,600)
Income available to common stockholders for basic earnings per share																	456,900	557,700	679,200
Effect of dilutive securities																		100
Income available to common stockholders for diluted earnings per share																	$ 456,900	$ 557,800	$ 679,200
Weighted average common shares outstanding applicable to basic earnings per share																	15,436,286	15,871,055	16,405,388
Effect of dilutive securities																	6,363	8,046
Adjusted weighted average common shares outstanding applicable to diluted earnings per share																	15,442,649	15,879,101	16,405,388

[1] Attributable to Alleghany stockholders.