Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2020		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 2,663,500		$ 2,522,900		$ 2,227,400		$ 1,482,900		$ 2,300,800		$ 2,159,900		$ 2,258,700		$ 2,321,200		$ 8,896,719	$ 9,040,716	$ 6,887,160
Net earnings (losses)	$ 159,100	[1]	$ 126,500	[1]	$ 177,400	[1]	$ (361,200)	[1]	$ 31,700	[1]	$ 90,400	[1]	$ 295,500	[1]	$ 440,200	[1]	$ 101,754	$ 857,801	$ 39,539
Basic earnings (losses) per share of common stock	$ 11.28	[1],[2]	$ 8.86	[1],[2]	$ 12.39	[1],[2]	$ (25.19)	[1],[2]	$ 2.20	[1],[2]	$ 6.27	[1],[2]	$ 20.46	[1],[2]	$ 30.40	[1],[2]	$ 7.14	$ 59.44	$ 2.62

[1] Attributable to Alleghany stockholders.

[2] Earnings per share by quarter may not equal the amount for the full year due to the timing of repurchases of Common Stock, as well as rounding.