Condensed Statements of Earnings (Detail) - USD ($) $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2021		Sep. 30, 2021		Jun. 30, 2021		Mar. 31, 2021		Dec. 31, 2020		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues
Net investment income																	$ 540,445	$ 490,856	$ 550,241
Change in the fair value of equity securities																	506,769	(110,459)	709,695
Net realized capital gains																	67,493	3,098	(6,551)
Change in allowance for credit losses on available for sale securities																	2,077	(8,029)
Change in allowance for credit losses on available for sale securities																			(19,660)
Product and service revenues																	3,789,668	2,521,092	2,328,848
Total revenues	$ 3,555,000		$ 2,866,600		$ 2,928,700		$ 2,653,700		$ 2,663,500		$ 2,522,900		$ 2,227,400		$ 1,482,900		12,004,105	8,896,719	9,040,716
Costs and Expenses
Interest expense																	102,308	88,152	99,957
Corporate administration																	57,218	48,581	74,830
Total costs and expenses																	10,591,402	8,739,102	7,917,080
Earnings (losses) before income taxes																	1,412,703	157,617	1,123,636
Income taxes																	281,925	30,734	233,435
Net earnings																	1,130,778	126,883	890,201
Net earnings attributable to noncontrolling interest																	95,886	25,129	32,400
Net earnings attributable to Alleghany stockholders	$ 516,200	[1]	$ (115,000)	[1]	$ 403,700	[1]	$ 230,000	[1]	$ 159,100	[1]	$ 126,500	[1]	$ 177,400	[1]	$ (361,200)	[1]	1,034,892	101,754	857,801
Parent Company
Revenues
Net investment income																	32,778	24,448	10,790
Change in the fair value of equity securities																	49,715	(54,647)	3,937
Net realized capital gains																	2,322	6,522	(13,497)
Change in allowance for credit losses on available for sale securities																	57	(57)
Change in allowance for credit losses on available for sale securities																			0
Product and service revenues																	81	771	85
Total revenues																	84,953	(22,963)	1,315
Costs and Expenses
Interest expense																	60,379	46,811	52,908
Corporate administration																	57,301	48,849	70,692
Total costs and expenses																	117,680	95,660	123,600
(Losses) before equity in earnings of consolidated subsidiaries and income taxes																	(32,727)	(118,623)	(122,285)
Equity in earnings of consolidated subsidiaries																	1,445,430	276,240	1,245,921
Earnings (losses) before income taxes																	1,412,703	157,617	1,123,636
Income taxes																	281,925	30,734	233,435
Net earnings																	1,130,778	126,883	890,201
Net earnings attributable to noncontrolling interest																	95,886	25,129	32,400
Net earnings attributable to Alleghany stockholders																	$ 1,034,892	$ 101,754	$ 857,801

[1] Attributable to Alleghany stockholders.