Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Gross premiums written		$ 2,238,000	$ 2,219,900	$ 4,385,000	$ 4,270,200
Net premiums written		1,830,000	1,878,200	3,622,800	3,633,000
Net premiums earned		1,715,188	1,784,351	3,320,646	3,389,114
Net loss and LAE		1,039,127	1,087,206	1,980,914	2,199,276
Commissions, brokerage and other underwriting expenses		510,935	523,513	988,100	999,549
Underwriting profit (loss)	[1]	165,200	173,700	351,600	190,300
Net investment income		105,452	126,931	218,928	280,415
Change in the fair value of equity securities		(500,223)	203,902	(639,000)	316,630
Net realized capital gains		11,167	12,942	1,164	25,873
Change in allowance for credit losses on available for sale securities		(7,595)	246	(8,177)	2,218
Product and service revenues		1,272,902	800,425	2,422,064	1,568,260
Other operating expenses		1,149,986	739,592	2,178,878	1,463,481
Corporate administration		17,278	20,148	28,001	29,706
Amortization of intangible assets		14,941	12,431	29,023	23,909
Interest expense		28,544	24,037	60,654	47,789
(Losses) earnings before income taxes		(163,920)	521,870	50,055	818,800
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		2,247,800	2,227,500	4,405,700	4,288,800
Net premiums written		1,830,000	1,878,200	3,622,800	3,633,000
Net premiums earned		1,715,200	1,784,400	3,320,600	3,389,100
Net loss and LAE		1,039,100	1,087,200	1,980,900	2,199,300
Commissions, brokerage and other underwriting expenses		510,900	523,500	988,100	999,500
Underwriting profit (loss)	[1]	165,200	173,700	351,600	190,300
Net investment income		117,700	136,600	226,100	259,700
Change in the fair value of equity securities		(405,100)	169,500	(559,200)	254,800
Net realized capital gains		11,100	13,000	1,300	24,500
Change in allowance for credit losses on available for sale securities		(7,600)	200	(8,200)	2,100
Product and service revenues		1,268,900	800,300	2,398,300	1,568,100
Other operating expenses		1,149,500	739,200	2,176,300	1,462,900
Corporate administration		0	400	0	(200)
Amortization of intangible assets		14,900	12,400	29,000	23,900
Interest expense		12,000	10,400	24,500	20,900
(Losses) earnings before income taxes		(26,200)	530,900	180,100	792,000
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,429,400	1,546,500	2,957,700	3,084,700
Net premiums written		1,280,300	1,409,100	2,638,300	2,804,000
Net premiums earned		1,259,100	1,393,200	2,421,900	2,623,000
Net loss and LAE		773,600	843,700	1,496,600	1,692,700
Commissions, brokerage and other underwriting expenses		397,100	425,300	765,400	801,200
Underwriting profit (loss)	[1]	88,400	124,200	159,900	129,100
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		380,300	505,400	834,700	1,070,700
Net premiums written		286,400	419,900	633,000	878,900
Net premiums earned		305,800	438,900	597,300	810,200
Net loss and LAE		177,400	273,700	339,500	597,200
Commissions, brokerage and other underwriting expenses		92,200	123,700	178,900	232,900
Underwriting profit (loss)	[1]	36,200	41,500	78,900	(19,900)
Operating Segments | Reinsurance Segment | Casualty & Specialty
Segment Reporting Information [Line Items]
Gross premiums written	[2]	1,049,100	1,041,100	2,123,000	2,014,000
Net premiums written	[2]	993,900	989,200	2,005,300	1,925,100
Net premiums earned	[2]	953,300	954,300	1,824,600	1,812,800
Net loss and LAE	[2]	596,200	570,000	1,157,100	1,095,500
Commissions, brokerage and other underwriting expenses	[2]	304,900	301,600	586,500	568,300
Underwriting profit (loss)	[1],[2]	52,200	82,700	81,000	149,000
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		818,400	681,000	1,448,000	1,204,100
Net premiums written		549,700	469,100	984,500	829,000
Net premiums earned		456,100	391,200	898,700	766,100
Net loss and LAE		265,500	243,500	484,300	506,600
Commissions, brokerage and other underwriting expenses		113,800	98,200	222,700	198,300
Underwriting profit (loss)	[1]	76,800	49,500	191,700	61,200
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		686,800	557,600	1,204,300	978,400
Net premiums written		441,800	363,700	783,800	636,200
Net premiums earned		360,400	296,300	706,800	579,900
Net loss and LAE		202,400	186,900	361,800	395,100
Commissions, brokerage and other underwriting expenses		76,300	60,600	148,500	124,700
Underwriting profit (loss)	[1]	81,700	48,800	196,500	60,100
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		131,600	123,400	243,700	225,700
Net premiums written		107,900	105,400	200,700	192,800
Net premiums earned		95,700	94,900	191,900	186,200
Net loss and LAE		63,100	56,600	122,500	111,500
Commissions, brokerage and other underwriting expenses		37,500	37,600	74,200	73,600
Underwriting profit (loss)	[1]	(4,900)	700	(4,800)	1,100
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		2,247,800	2,227,500	4,405,700	4,288,800
Net premiums written		1,830,000	1,878,200	3,622,800	3,633,000
Net premiums earned		1,715,200	1,784,400	3,320,600	3,389,100
Net loss and LAE		1,039,100	1,087,200	1,980,900	2,199,300
Commissions, brokerage and other underwriting expenses		510,900	523,500	988,100	999,500
Underwriting profit (loss)	[1]	165,200	173,700	351,600	190,300
Net investment income		117,800	136,600	226,100	259,700
Change in the fair value of equity securities		(405,100)	169,500	(559,200)	254,800
Net realized capital gains		3,000	12,800	(12,900)	23,300
Change in allowance for credit losses on available for sale securities		(7,600)	200	(8,200)	2,100
Product and service revenues		6,600	11,100	14,700	19,700
Other operating expenses		11,100	15,100	24,600	29,500
Corporate administration		0	400	0	(200)
Amortization of intangible assets		300	800	700	1,000
Interest expense		6,700	6,700	13,500	13,400
(Losses) earnings before income taxes		(138,200)	480,900	(26,700)	706,200
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 profit (loss)	[1]	0	0	0	0
Net investment income		(100)	0	0	0
Change in the fair value of equity securities		0	0	0	0
Net realized capital gains		8,100	200	14,200	1,200
Change in allowance for credit losses on available for sale securities		0	0	0	0
Product and service revenues		1,262,300	789,200	2,383,600	1,548,400
Other operating expenses		1,138,400	724,100	2,151,700	1,433,400
Corporate administration		0	0	0	0
Amortization of intangible assets		14,600	11,600	28,300	22,900
Interest expense		5,300	3,700	11,000	7,500
(Losses) earnings before income taxes		112,000	50,000	206,800	85,800
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written		(9,800)	(7,600)	(20,700)	(18,600)
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 profit (loss)	[1]	0	0	0	0
Net investment income		(12,200)	(9,700)	(7,200)	20,700
Change in the fair value of equity securities		(95,100)	34,400	(79,800)	61,800
Net realized capital gains		0	(100)	(100)	1,400
Change in allowance for credit losses on available for sale securities		0	0	0	100
Product and service revenues		4,000	100	23,800	200
Other operating expenses		600	300	2,500	600
Corporate administration		17,300	19,800	28,000	29,900
Amortization of intangible assets		0	0	0	0
Interest expense		16,500	13,600	36,200	26,900
(Losses) earnings before income taxes		$ (137,700)	$ (9,000)	$ (130,000)	$ 26,800

[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.