Reconciliation of Earnings and Share Data used in Basic and Diluted Earnings per Share Computations (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share Disclosure [Line Items]
Net earnings available to Alleghany stockholders	$ 155,831	$ 96,530	$ 387,407	$ 404,217
Effect of dilutive securities				100
Income available to common stockholders for diluted earnings per share	$ 155,800	$ 96,500	$ 387,400	$ 404,300
Weighted average common shares outstanding applicable to basic earnings per share	15,438,399	15,900,759	15,443,150	15,969,984
Effect of dilutive securities			6,330	8,155
Adjusted weighted average common shares outstanding applicable to diluted earnings per share	15,438,399	15,900,759	15,449,480	15,978,139