Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues
Net investment income																	$ 550,241	$ 500,534	$ 451,016
Change in the fair value of equity securities																	709,695	(228,994)
Net realized capital gains																	(6,551)	(3,241)	107,222
Other than temporary impairment losses																	(19,660)	(1,328)
Other than temporary impairment losses																			(16,871)
Noninsurance revenue																	2,328,848	1,643,999	928,298
Total revenues	$ 2,300,800		$ 2,159,900		$ 2,258,700		$ 2,321,200		$ 1,227,100		$ 2,177,400		$ 1,897,700		$ 1,585,000		9,040,716	6,887,160	6,424,655
Costs and Expenses
Interest expense																	99,957	90,724	83,070
Corporate administration																	74,830	15,732	46,998
Total costs and expenses																	7,917,080	6,847,568	6,387,965
Earnings (losses) before income taxes																	1,123,636	39,592	36,690
Income taxes																	233,435	(15,062)	(63,802)
Net earnings																	890,201	54,654	100,492
Net earnings attributable to noncontrolling interests																	32,400	15,115	10,359
Net earnings attributable to Alleghany stockholders	$ 31,700	[1]	$ 90,400	[1]	$ 295,500	[1]	$ 440,200	[1]	$ (712,100)	[1]	$ 284,900	[1]	$ 295,100	[1]	$ 171,600	[1]	857,801	39,539	90,133
Parent Company
Revenues
Net investment income																	10,790	16,548	13,659
Change in the fair value of equity securities																	3,937	(44,780)
Net realized capital gains																	(13,497)	(265)	3,326
Noninsurance revenue																	85	64	683
Total revenues																	1,315	(28,433)	17,668
Costs and Expenses
Interest expense																	52,908	53,121	52,103
Corporate administration																	70,692	17,565	45,308
Total costs and expenses																	123,600	70,686	97,411
(Losses) before equity in earnings of consolidated subsidiaries and income taxes																	(122,285)	(99,119)	(79,743)
Equity in earnings of consolidated subsidiaries																	1,245,921	138,711	116,433
Earnings (losses) before income taxes																	1,123,636	39,592	36,690
Income taxes																	233,435	(15,062)	(63,802)
Net earnings																	890,201	54,654	100,492
Net earnings attributable to noncontrolling interests																	32,400	15,115	10,359
Net earnings attributable to Alleghany stockholders																	$ 857,801	$ 39,539	$ 90,133

[1] Attributable to Alleghany stockholders.