Reconciliation of Earnings and Share Data used in Basic and Diluted (Losses) Earnings per Share Computations (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[1]	Sep. 30, 2016	[1]	Jun. 30, 2016	[1]	Mar. 31, 2016	[1]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share Disclosure [Line Items]
Net earnings available to Alleghany stockholders	$ 153,300		$ (314,200)		$ 101,800		$ 149,200		$ 69,500		$ 155,800		$ 77,100		$ 154,500		$ 90,133	$ 456,921	$ 560,315
Adjustment related to redeemable noncontrolling interests																			(2,600)
Income available to common stockholders for basic earnings per share																	90,100	456,900	557,700
Effect of dilutive securities																			100
Income available to common stockholders for diluted earnings per share																	$ 90,100	$ 456,900	$ 557,800
Weighted average common shares outstanding applicable to basic earnings per share																	15,410,034	15,436,286	15,871,055
Effect of dilutive securities																		6,363	8,046
Adjusted weighted average common shares outstanding applicable to diluted earnings per share																	15,410,034	15,442,649	15,879,101

[1] Attributable to Alleghany stockholders.