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, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share Disclosure [Line Items]
Net earnings available to Alleghany stockholders		$ (712,100)		$ 284,900		$ 295,100		$ 171,600		$ 153,300		$ (314,200)		$ 101,800		$ 149,200		$ 39,539	$ 90,133	$ 456,921
Effect of dilutive securities																		0	0	0
Income available to common stockholders for diluted earnings per share																		$ 39,500	$ 90,100	$ 456,900
Weighted average common shares outstanding applicable to basic earnings per share																		15,062,567	15,410,034	15,436,286
Effect of dilutive securities																				6,363
Adjusted weighted average common shares outstanding applicable to diluted earnings per share																		15,062,567	15,410,034	15,442,649
Contingently issuable shares	[2]																	23,947	78,345	68,429

[1] Attributable to Alleghany stockholders.

[2] Contingently issuable shares were potentially available in the periods presented, but were not included in the diluted earnings per share computations because the impact was anti-dilutive to the earnings per share calculation.