Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Gross premiums written		$ 1,891,400	$ 1,651,100	$ 5,383,100	$ 4,928,500
Net premiums written		1,637,100	1,413,900	4,648,800	4,246,000
Net premiums earned		1,556,859	1,389,981	4,396,147	4,043,298
Net loss and LAE		1,173,655	907,736	3,225,461	2,497,037
Commissions, brokerage and other underwriting expenses		464,522	449,519	1,315,935	1,313,961
Underwriting (loss) profit	[1]	(81,300)	32,800	(145,300)	232,300
Net investment income		129,265	147,829	359,938	413,623
Change in the fair value of equity securities		92,799	(16,691)	(187,991)	519,322
Net realized capital gains		16,751	3,957	(8,323)	20,753
Change in allowance for credit losses on available for sale securities		3,454		(10,900)
Change in allowance for credit losses on available for sale securities			(3,597)		(13,617)
Noninsurance revenue		723,785	638,478	1,684,335	1,756,507
Other operating expenses		669,423	617,326	1,646,390	1,701,218
Corporate administration		12,871	22,276	17,143	67,612
Amortization of intangible assets		11,223	8,095	32,884	23,790
Interest expense		23,319	25,703	63,869	74,363
(Losses) earnings before income taxes		167,900	129,302	(68,476)	1,061,905
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		1,900,600	1,658,600	5,409,200	4,949,000
Net premiums written		1,637,100	1,413,900	4,648,800	4,246,000
Net premiums earned		1,556,900	1,390,000	4,396,100	4,043,300
Net loss and LAE		1,173,700	907,700	3,225,500	2,497,000
Commissions, brokerage and other underwriting expenses		464,500	449,500	1,315,900	1,314,000
Underwriting (loss) profit	[1]	(81,300)	32,800	(145,300)	232,300
Net investment income		122,700	146,300	349,600	404,900
Change in the fair value of equity securities		103,400	(16,700)	(176,000)	515,700
Net realized capital gains		15,600	3,900	72,100	20,700
Change in allowance for credit losses on available for sale securities		3,400		(10,700)
Change in allowance for credit losses on available for sale securities			(3,600)		(13,600)
Noninsurance revenue		722,700	635,500	1,678,700	1,746,300
Other operating expenses		666,800	610,700	1,634,200	1,680,300
Corporate administration		800	1,500	(900)	4,400
Amortization of intangible assets		11,200	8,100	32,900	23,800
Interest expense		10,100	12,000	31,700	34,500
(Losses) earnings before income taxes		197,600	165,900	70,500	1,163,300
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,378,000	1,243,300	3,859,400	3,685,600
Net premiums written		1,263,200	1,111,700	3,560,700	3,332,100
Net premiums earned		1,207,300	1,097,600	3,411,300	3,201,700
Net loss and LAE		859,300	730,700	2,499,800	2,028,900
Commissions, brokerage and other underwriting expenses		371,400	360,900	1,044,200	1,050,800
Underwriting (loss) profit	[1]	(23,400)	6,000	(132,700)	122,000
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		473,300	455,900	1,299,200	1,252,600
Net premiums written		375,900	339,000	1,044,600	961,100
Net premiums earned		374,700	327,000	1,024,800	939,100
Net loss and LAE		306,100	225,100	827,400	523,800
Commissions, brokerage and other underwriting expenses		112,400	108,300	314,600	315,800
Underwriting (loss) profit	[1]	(43,800)	(6,400)	(117,200)	99,500
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[2]	904,700	787,400	2,560,200	2,433,000
Net premiums written	[2]	887,300	772,700	2,516,100	2,371,000
Net premiums earned	[2]	832,600	770,600	2,386,500	2,262,600
Net loss and LAE	[2]	553,200	505,600	1,672,400	1,505,100
Commissions, brokerage and other underwriting expenses	[2]	259,000	252,600	729,600	735,000
Underwriting (loss) profit	[1],[2]	20,400	12,400	(15,500)	22,500
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		522,600	415,300	1,549,800	1,263,400
Net premiums written		373,900	302,200	1,088,100	913,900
Net premiums earned		349,600	292,400	984,800	841,600
Net loss and LAE		314,400	177,000	725,700	468,100
Commissions, brokerage and other underwriting expenses		93,100	88,600	271,700	263,200
Underwriting (loss) profit	[1]	(57,900)	26,800	(12,600)	110,300
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		410,800	319,700	1,251,300	991,100
Net premiums written		271,900	213,400	816,400	661,700
Net premiums earned		260,400	208,900	731,300	603,700
Net loss and LAE		256,600	125,900	568,500	325,800
Commissions, brokerage and other underwriting expenses		58,800	55,700	169,400	167,200
Underwriting (loss) profit	[1]	(55,000)	27,300	(6,600)	110,700
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		111,800	95,600	298,500	272,300
Net premiums written		102,000	88,800	271,700	252,200
Net premiums earned		89,200	83,500	253,500	237,900
Net loss and LAE		57,800	51,100	157,200	142,300
Commissions, brokerage and other underwriting expenses		34,300	32,900	102,300	96,000
Underwriting (loss) profit	[1]	(2,900)	(500)	(6,000)	(400)
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,900,600	1,658,600	5,409,200	4,949,000
Net premiums written		1,637,100	1,413,900	4,648,800	4,246,000
Net premiums earned		1,556,900	1,390,000	4,396,100	4,043,300
Net loss and LAE		1,173,700	907,700	3,225,500	2,497,000
Commissions, brokerage and other underwriting expenses		464,500	449,500	1,315,900	1,314,000
Underwriting (loss) profit	[1]	(81,300)	32,800	(145,300)	232,300
Net investment income		122,700	145,100	347,800	401,500
Change in the fair value of equity securities		103,400	(16,700)	(176,000)	515,700
Net realized capital gains		1,700	4,100	36,800	20,400
Change in allowance for credit losses on available for sale securities		3,400		(10,700)
Change in allowance for credit losses on available for sale securities			(3,600)		(13,600)
Noninsurance revenue		8,900	7,500	24,400	19,500
Other operating expenses		23,400	32,500	66,500	88,300
Corporate administration		800	1,500	(900)	4,400
Amortization of intangible assets		200	400	600	1,000
Interest expense		6,800	6,800	20,300	20,300
(Losses) earnings before income taxes		127,600	128,000	(9,500)	1,061,800
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		0	1,200	1,800	3,400
Change in the fair value of equity securities		0	0	0	0
Net realized capital gains		13,900	(200)	35,300	300
Change in allowance for credit losses on available for sale securities		0		0
Change in allowance for credit losses on available for sale securities			0		0
Noninsurance revenue		713,800	628,000	1,654,300	1,726,800
Other operating expenses		643,400	578,200	1,567,700	1,592,000
Corporate administration		0	0	0	0
Amortization of intangible assets		11,000	7,700	32,300	22,800
Interest expense		3,300	5,200	11,400	14,200
(Losses) earnings before income taxes		70,000	37,900	80,000	101,500
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written		(9,200)	(7,500)	(26,100)	(20,500)
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		6,600	1,500	10,300	8,700
Change in the fair value of equity securities		(10,600)	0	(12,000)	3,600
Net realized capital gains		1,100	0	(80,400)	100
Change in allowance for credit losses on available for sale securities		0		(200)
Change in allowance for credit losses on available for sale securities			0		0
Noninsurance revenue		1,100	3,000	5,700	10,200
Other operating expenses		2,600	6,600	12,200	20,900
Corporate administration		12,100	20,800	18,000	63,200
Amortization of intangible assets		0	0	0	0
Interest expense		13,200	13,700	32,200	39,900
(Losses) earnings before income taxes		$ (29,700)	$ (36,600)	$ (139,000)	$ (101,400)

[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, noninsurance revenue, 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.