Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2020		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenues
Net investment income																	$ 490,856	$ 550,241	$ 500,534
Change in the fair value of equity securities																	(110,459)	709,695	(228,994)
Net realized capital gains																	3,098	(6,551)	(3,241)
Change in allowance for credit losses on available for sale securities																	(8,029)
Change in allowance for credit losses on available for sale securities																		(19,660)	(1,328)
Noninsurance revenue																	2,521,092	2,328,848	1,643,999
Total revenues	$ 2,663,500		$ 2,522,900		$ 2,227,400		$ 1,482,900		$ 2,300,800		$ 2,159,900		$ 2,258,700		$ 2,321,200		8,896,719	9,040,716	6,887,160
Costs and Expenses
Interest expense																	88,152	99,957	90,724
Corporate administration																	48,581	74,830	15,732
Total costs and expenses																	8,739,102	7,917,080	6,847,568
Earnings (losses) before income taxes																	157,617	1,123,636	39,592
Income taxes																	30,734	233,435	(15,062)
Net earnings																	126,883	890,201	54,654
Net earnings attributable to noncontrolling interests																	25,129	32,400	15,115
Net earnings attributable to Alleghany stockholders	$ 159,100	[1]	$ 126,500	[1]	$ 177,400	[1]	$ (361,200)	[1]	$ 31,700	[1]	$ 90,400	[1]	$ 295,500	[1]	$ 440,200	[1]	101,754	857,801	39,539
Parent Company
Revenues
Net investment income																	24,448	10,790	16,548
Change in the fair value of equity securities																	(54,647)	3,937	(44,780)
Net realized capital gains																	6,522	(13,497)	(265)
Change in allowance for credit losses on available for sale securities																	(57)
Change in allowance for credit losses on available for sale securities																		0	0
Noninsurance revenue																	771	85	64
Total revenues																	(22,963)	1,315	(28,433)
Costs and Expenses
Interest expense																	46,811	52,908	53,121
Corporate administration																	48,849	70,692	17,565
Total costs and expenses																	95,660	123,600	70,686
(Losses) before equity in earnings of consolidated subsidiaries and income taxes																	(118,623)	(122,285)	(99,119)
Equity in earnings of consolidated subsidiaries																	276,240	1,245,921	138,711
Earnings (losses) before income taxes																	157,617	1,123,636	39,592
Income taxes																	30,734	233,435	(15,062)
Net earnings																	126,883	890,201	54,654
Net earnings attributable to noncontrolling interests																	25,129	32,400	15,115
Net earnings attributable to Alleghany stockholders																	$ 101,754	$ 857,801	$ 39,539

[1] Attributable to Alleghany stockholders.