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, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net earnings available to Alleghany stockholders		$ 90,432	$ 295,459	$ 440,227	$ 284,900	$ 295,116	$ 171,575	$ 826,118	$ 751,591
Income available to common stockholders for diluted earnings per share		$ 90,400			$ 284,900			$ 826,100	$ 751,600
Weighted average common shares outstanding applicable to basic earnings per share		14,422,581			14,937,135			14,447,794	15,168,831
Effect of dilutive securities								10,956	4,849
Adjusted weighted average common shares outstanding applicable to diluted earnings per share		14,422,581			14,937,135			14,458,750	15,173,680
Contingently issuable shares	[1]							50,556	61,285

[1] 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.