Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Net investment income																	$ 438,817	$ 459,876	$ 465,664
Net realized capital gains																	213,897	247,058	232,119
Other than temporary impairment losses																	(133,868)	(36,294)	(44,047)
Other income																	250,346	150,522	78,702
Total revenues	$ 1,352,400		$ 1,189,100		$ 1,300,500		$ 1,157,600		$ 1,302,100		$ 1,351,900		$ 1,291,200		$ 1,286,600		4,999,478	5,231,809	4,971,654
Costs and Expenses
Interest expense																	91,778	90,052	86,740
Corporate administration																	46,503	47,054	36,111
Total costs and expenses																	4,242,110	4,299,900	4,116,418
Earnings (losses) before income taxes																	757,368	931,909	855,236
Income taxes																	195,173	251,777	225,882
Net earnings																	562,195	680,132	629,354
Net earnings attributable to noncontrolling interest																	1,880	893	933
Net earnings attributable to Alleghany stockholders	$ 156,100	[1]	$ 96,500	[1]	$ 182,500	[1]	$ 125,200	[1]	$ 139,100	[1]	$ 186,300	[1]	$ 149,000	[1]	$ 204,900	[1]	560,315	679,239	628,421
Parent Company
Revenues
Net investment income																	5,800	8,169	7,540
Net realized capital gains																	(9,088)	14,349	(4,769)
Other than temporary impairment losses																	(2,388)
Other income																	218	265	188
Total revenues																	(5,458)	22,783	2,959
Costs and Expenses
Interest expense																	52,056	42,310	37,302
Corporate administration																	45,573	45,741	36,111
Total costs and expenses																	97,629	88,051	73,413
Operating (losses)																	(103,087)	(65,268)	(70,454)
Equity in earnings of consolidated subsidiaries																	860,455	997,177	925,690
Earnings (losses) before income taxes																	757,368	931,909	855,236
Income taxes																	195,173	251,777	225,882
Net earnings																	562,195	680,132	629,354
Net earnings attributable to noncontrolling interest																	1,880	893	933
Net earnings attributable to Alleghany stockholders																	$ 560,315	$ 679,239	$ 628,421

[1] Attributable to Alleghany stockholders.