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, 2020	[1]	Sep. 30, 2020	[1]	Jun. 30, 2020	[1]	Mar. 31, 2020	[1]	Dec. 31, 2019	[1]	Sep. 30, 2019	[1]	Jun. 30, 2019	[1]	Mar. 31, 2019	[1]	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net earnings available to Alleghany stockholders		$ 159,100		$ 126,500		$ 177,400		$ (361,200)		$ 31,700		$ 90,400		$ 295,500		$ 440,200		$ 101,754	$ 857,801	$ 39,539
Effect of dilutive securities																		(1,200)	0	0
Income available to common stockholders for diluted earnings per share																		$ 100,600	$ 857,800	$ 39,500
Weighted average common shares outstanding applicable to basic earnings per share																		14,257,793	14,431,892	15,062,567
Effect of dilutive securities																		25,825	11,584	0
Adjusted weighted average common shares outstanding applicable to diluted earnings per share																		14,283,618	14,443,476	15,062,567
Contingently issuable shares	[2]																	34,148	48,468	23,947

[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.