Selected Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Selected Quarterly Financial Data [Line Items]
Revenues	$ 1,362,500		$ 1,189,500		$ 1,196,200		$ 1,223,600		$ 1,240,400		$ 1,229,500		$ 1,234,400		$ 1,048,900		$ 4,971,654	$ 4,753,212	$ 981,837
Net earnings	$ 205,300	[1]	$ 113,200	[1]	$ 113,700	[1]	$ 196,300	[1]	$ (92,600)	[1]	$ 125,400	[1]	$ 109,300	[1]	$ 560,100	[1]	$ 628,421	$ 702,244	$ 143,251
Basic earnings per share of Common Stock	$ 12.24	[1],[2]	$ 6.75	[1],[2]	$ 6.78	[1],[2]	$ 11.67	[1],[2]	$ (5.47)	[1],[2]	$ 7.41	[1],[2]	$ 6.46	[1],[2]	$ 51.17	[1],[2]	$ 37.44	$ 45.48	$ 16.26

[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 both the shares issued in connection with the merger in 2012 and repurchases of Common Stock, as well as rounding.