Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Gross premiums written		$ 2,378,000	$ 1,891,400	$ 6,648,300	$ 5,383,100
Net premiums written		1,983,100	1,637,100	5,616,100	4,648,800
Net premiums earned		1,842,894	1,556,859	5,232,008	4,396,147
Net loss and LAE		1,513,877	1,173,655	3,713,153	3,225,461
Commissions, brokerage and other underwriting expenses		529,247	464,522	1,528,796	1,315,935
Underwriting (loss) profit	[1]	(200,200)	(81,300)	(10,000)	(145,300)
Net investment income		132,965	129,265	413,380	359,938
Change in the fair value of equity securities		(136,042)	92,799	180,588	(187,991)
Net realized capital gains		20,172	16,751	46,045	(8,323)
Change in allowance for credit losses on available for sale securities		54	3,454	2,272	(10,900)
Product and service revenues		1,006,570	723,785	2,574,830	1,684,335
Other operating expenses		890,972	669,423	2,354,453	1,646,390
Corporate administration		6,225	12,871	35,931	17,143
Amortization of intangible assets		12,637	11,223	36,546	32,884
Interest expense		25,659	23,319	73,448	63,869
(Losses) earnings before income taxes		(112,004)	167,900	706,796	(68,476)
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		2,389,700	1,900,600	6,678,500	5,409,200
Net premiums written		1,983,100	1,637,100	5,616,100	4,648,800
Net premiums earned		1,842,900	1,556,900	5,232,000	4,396,100
Net loss and LAE		1,513,900	1,173,700	3,713,200	3,225,500
Commissions, brokerage and other underwriting expenses		529,200	464,500	1,528,800	1,315,900
Underwriting (loss) profit	[1]	(200,200)	(81,300)	(10,000)	(145,300)
Net investment income		125,100	122,700	384,800	349,600
Change in the fair value of equity securities		(78,300)	103,400	176,600	(176,000)
Net realized capital gains		20,200	15,600	44,700	72,100
Change in allowance for credit losses on available for sale securities		100	3,400	2,200	(10,700)
Product and service revenues		996,800	722,700	2,564,800	1,678,700
Other operating expenses		889,800	666,800	2,352,800	1,634,200
Corporate administration		(100)	800	(500)	(900)
Amortization of intangible assets		12,600	11,200	36,500	32,900
Interest expense		10,600	10,100	31,500	31,700
(Losses) earnings before income taxes		(49,200)	197,600	742,800	70,500
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,728,200	1,378,000	4,812,900	3,859,400
Net premiums written		1,514,600	1,263,200	4,318,600	3,560,700
Net premiums earned		1,423,300	1,207,300	4,046,300	3,411,300
Net loss and LAE		1,230,800	859,300	2,923,500	2,499,800
Commissions, brokerage and other underwriting expenses		424,400	371,400	1,225,600	1,044,200
Underwriting (loss) profit	[1]	(231,900)	(23,400)	(102,800)	(132,700)
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		617,200	473,300	1,687,900	1,299,200
Net premiums written		467,900	375,900	1,346,800	1,044,600
Net premiums earned		474,300	374,700	1,284,500	1,024,800
Net loss and LAE		611,200	306,100	1,208,400	827,400
Commissions, brokerage and other underwriting expenses		124,000	112,400	356,900	314,600
Underwriting (loss) profit	[1]	(260,900)	(43,800)	(280,800)	(117,200)
Operating Segments | Reinsurance Segment | Casualty & Specialty
Segment Reporting Information [Line Items]
Gross premiums written	[2]	1,111,000	904,700	3,125,000	2,560,200
Net premiums written	[2]	1,046,700	887,300	2,971,800	2,516,100
Net premiums earned	[2]	949,000	832,600	2,761,800	2,386,500
Net loss and LAE	[2]	619,600	553,200	1,715,100	1,672,400
Commissions, brokerage and other underwriting expenses	[2]	300,400	259,000	868,700	729,600
Underwriting (loss) profit	[1],[2]	29,000	20,400	178,000	(15,500)
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		661,500	522,600	1,865,600	1,549,800
Net premiums written		468,500	373,900	1,297,500	1,088,100
Net premiums earned		419,600	349,600	1,185,700	984,800
Net loss and LAE		283,100	314,400	789,700	725,700
Commissions, brokerage and other underwriting expenses		104,800	93,100	303,200	271,700
Underwriting (loss) profit	[1]	31,700	(57,900)	92,800	(12,600)
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		533,100	410,800	1,511,500	1,251,300
Net premiums written		358,000	271,900	994,200	816,400
Net premiums earned		316,600	260,400	896,500	731,300
Net loss and LAE		219,500	256,600	614,500	568,500
Commissions, brokerage and other underwriting expenses		66,500	58,800	191,300	169,400
Underwriting (loss) profit	[1]	30,600	(55,000)	90,700	(6,600)
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		128,400	111,800	354,100	298,500
Net premiums written		110,500	102,000	303,300	271,700
Net premiums earned		103,000	89,200	289,200	253,500
Net loss and LAE		63,600	57,800	175,200	157,200
Commissions, brokerage and other underwriting expenses		38,300	34,300	111,900	102,300
Underwriting (loss) profit	[1]	1,100	(2,900)	2,100	(6,000)
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		2,389,700	1,900,600	6,678,500	5,409,200
Net premiums written		1,983,100	1,637,100	5,616,100	4,648,800
Net premiums earned		1,842,900	1,556,900	5,232,000	4,396,100
Net loss and LAE		1,513,900	1,173,700	3,713,200	3,225,500
Commissions, brokerage and other underwriting expenses		529,200	464,500	1,528,800	1,315,900
Underwriting (loss) profit	[1]	200,200	(81,300)	(10,000)	(145,300)
Net investment income		125,000	122,700	384,700	347,800
Change in the fair value of equity securities		(78,300)	103,400	176,600	(176,000)
Net realized capital gains		20,700	1,700	44,000	36,800
Change in allowance for credit losses on available for sale securities		100	3,400	2,200	(10,700)
Product and service revenues		10,400	8,900	30,100	24,400
Other operating expenses		20,800	23,400	50,400	66,500
Corporate administration		(100)	800	(500)	(900)
Amortization of intangible assets		900	200	1,800	600
Interest expense		6,700	6,800	20,200	20,300
(Losses) earnings before income taxes		(150,600)	127,600	555,700	(9,500)
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Gross premiums written		0	0	0	0
Net premiums written		0	0	0	0
Net premiums earned		0	0	0	0
Net loss and LAE		0	0	0	0
Commissions, brokerage and other underwriting expenses		0	0	0	0
Underwriting (loss) profit	[1]	0	0	0	0
Net investment income		100	0	100	1,800
Change in the fair value of equity securities		0	0	0	0
Net realized capital gains		(500)	13,900	700	35,300
Change in allowance for credit losses on available for sale securities		0	0	0	0
Product and service revenues		986,400	713,800	2,534,700	1,654,300
Other operating expenses		869,000	643,400	2,302,400	1,567,700
Corporate administration		0	0	0	0
Amortization of intangible assets		11,700	11,000	34,700	32,300
Interest expense		3,900	3,300	11,300	11,400
(Losses) earnings before income taxes		101,400	70,000	187,100	80,000
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written		(11,700)	(9,200)	(30,200)	(26,100)
Net premiums written		0	0	0	0
Net premiums earned		0	0	0	0
Net loss and LAE		0	0	0	0
Commissions, brokerage and other underwriting expenses		0	0	0	0
Underwriting (loss) profit	[1]	0	0	0	0
Net investment income		(7,900)	6,600	28,600	10,300
Change in the fair value of equity securities		(57,800)	(10,600)	4,000	(12,000)
Net realized capital gains		(100)	1,100	1,300	(80,400)
Change in allowance for credit losses on available for sale securities		0	0	100	(200)
Product and service revenues		9,800	1,100	10,000	5,700
Other operating expenses		1,200	2,600	1,700	12,200
Corporate administration		6,300	12,100	36,400	18,000
Amortization of intangible assets		0	0	0	0
Interest expense		15,100	13,200	41,900	32,200
(Losses) earnings before income taxes		$ (62,800)	$ (29,700)	$ (36,000)	$ (139,000)

[1] Underwriting profit represents net premiums earned less net loss and LAE and commissions, brokerage and other underwriting expenses, all as determined in accordance with GAAP, and does not include net investment income, change in the fair value of equity securities, net realized capital gains, change in allowance for credit losses on available for sale securities, product and service revenues, other operating expenses, corporate administration, amortization of intangible assets or interest expense. Underwriting profit does not replace earnings before income taxes determined in accordance with GAAP as a measure of profitability. Rather, Alleghany believes that underwriting profit enhances the understanding of its reinsurance and insurance segments’ operating results by highlighting net earnings attributable to their underwriting performance. Earnings before income taxes (a GAAP measure) may show a profit despite an underlying underwriting loss. Where underwriting losses persist over extended periods, a reinsurance or an insurance company’s ability to continue as an ongoing concern may be at risk. Therefore, Alleghany views underwriting profit as an important measure in the overall evaluation of performance.

[2] Primarily consists of the following reinsurance lines of business: directors’ and officers’ liability; errors and omissions liability; general liability; medical malpractice; ocean marine and aviation; auto liability; accident and health; mortgage reinsurance; surety; and credit.