Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Net investment income																	$ 451,016	$ 438,455	$ 438,817
Net realized capital gains																	107,222	63,205	213,897
Other than temporary impairment losses																	(16,871)	(45,165)	(133,868)
Other revenue																	928,298	698,747	250,346
Total revenues	$ 1,670,800		$ 1,667,500		$ 1,553,900		$ 1,532,400		$ 1,455,300		$ 1,614,600		$ 1,582,100		$ 1,478,900		6,424,655	6,131,019	4,999,478
Costs and Expenses
Interest expense																	83,070	81,599	91,778
Corporate administration																	46,998	42,960	46,503
Total costs and expenses																	6,387,965	5,483,214	4,242,110
(Losses) earnings before income taxes																	36,690	647,805	757,368
Income taxes																	(63,802)	187,141	195,173
Net earnings																	100,492	460,664	562,195
Net earnings attributable to noncontrolling interest																	10,359	3,743	1,880
Net earnings attributable to Alleghany stockholders	$ 153,300	[1]	$ (314,200)	[1]	$ 101,800	[1]	$ 149,200	[1]	$ 69,500	[1]	$ 155,800	[1]	$ 77,100	[1]	$ 154,500	[1]	90,133	456,921	560,315
Parent Company
Revenues
Net investment income																	13,659	7,688	5,800
Net realized capital gains																	3,326	(10,674)	(9,088)
Other than temporary impairment losses																			(2,388)
Other revenue																	683	159	218
Total revenues																	17,668	(2,827)	(5,458)
Costs and Expenses
Interest expense																	52,103	52,470	52,056
Corporate administration																	45,308	42,035	45,573
Total costs and expenses																	97,411	94,505	97,629
Operating (losses)																	(79,743)	(97,332)	(103,087)
Equity in earnings of consolidated subsidiaries																	116,433	745,137	860,455
(Losses) earnings before income taxes																	36,690	647,805	757,368
Income taxes																	(63,802)	187,141	195,173
Net earnings																	100,492	460,664	562,195
Net earnings attributable to noncontrolling interest																	10,359	3,743	1,880
Net earnings attributable to Alleghany stockholders																	$ 90,133	$ 456,921	$ 560,315

[1] Attributable to Alleghany stockholders.