Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Quarterly Financial Data [Line Items]
Revenues	$ 1,227,100		$ 2,177,400		$ 1,897,700		$ 1,585,000		$ 1,670,800		$ 1,667,500		$ 1,553,900		$ 1,532,400		$ 6,887,160	$ 6,424,655	$ 6,131,019
Net earnings (losses)	$ (712,100)	[1]	$ 284,900	[1]	$ 295,100	[1]	$ 171,600	[1]	$ 153,300	[1]	$ (314,200)	[1]	$ 101,800	[1]	$ 149,200	[1]	$ 39,539	$ 90,133	$ 456,921
Basic earnings (losses) per share of Common Stock	$ (48.30)	[1],[2]	$ 19.07	[1],[2]	$ 19.44	[1],[2]	$ 11.15	[1],[2]	$ 9.96	[1],[2]	$ (20.38)	[1],[2]	$ 6.60	[1],[2]	$ 9.68	[1],[2]	$ 2.62	$ 5.85	$ 29.60

[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.