Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Quarterly Financial Data [Line Items]
Revenues	$ 1,455,300		$ 1,614,600		$ 1,582,100		$ 1,478,900		$ 1,352,400		$ 1,189,100		$ 1,300,500		$ 1,157,600		$ 6,131,019	$ 4,999,478	$ 5,231,809
Net earnings	$ 69,500	[1]	$ 155,800	[1]	$ 77,100	[1]	$ 154,500	[1]	$ 156,100	[1]	$ 96,500	[1]	$ 182,500	[1]	$ 125,200	[1]	$ 456,921	$ 560,315	$ 679,239
Basic earnings per share of Common Stock	$ 4.51	[1],[2]	$ 10.09	[1],[2]	$ 4.99	[1],[2]	$ 10.00	[1],[2]	$ 9.86	[1],[2]	$ 6.07	[1],[2]	$ 11.41	[1],[2]	$ 7.82	[1],[2]	$ 29.60	$ 35.14	$ 41.40

[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.