Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Gross premiums written		$ 7,329,400	$ 6,656,400	$ 5,895,600
Net premiums written		6,344,400	5,751,700	5,048,400
Net premiums earned		6,000,161	5,478,143	4,976,190
Net loss and LAE		4,339,046	3,686,435	3,520,431
Commissions, brokerage and other underwriting expenses	[1]	1,789,820	1,758,698	1,617,333
Underwriting (loss) profit	[2]	(128,700)	33,000	(161,500)
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
Other operating expenses		2,429,262	2,263,326	1,579,309
Corporate administration		48,581	74,830	15,732
Amortization of intangible assets		44,241	33,834	24,039
Interest expense		88,152	99,957	90,724
Earnings (losses) before income taxes		157,617	1,123,636	39,592
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		7,363,000	6,684,100	5,921,800
Net premiums written		6,344,400	5,751,700	5,048,400
Net premiums earned		6,000,200	5,478,100	4,976,200
Net loss and LAE		4,339,100	3,686,400	3,520,400
Commissions, brokerage and other underwriting expenses	[1]	1,789,800	1,758,700	1,617,300
Underwriting (loss) profit	[2]	(128,700)	33,000	(161,500)
Net investment income		467,600	539,500	485,700
Change in the fair value of equity securities		(55,800)	705,800	(184,200)
Net realized capital gains		72,600	7,000	32,500
Change in allowance for credit losses on available for sale securities		(8,000)
Change in allowance for credit losses on available for sale securities			(19,700)	(1,300)
Noninsurance revenue		2,512,400	2,316,300	1,596,600
Other operating expenses		2,413,600	2,236,000	1,545,400
Corporate administration		(300)	4,100	(1,700)
Amortization of intangible assets		44,200	33,800	24,000
Interest expense		42,500	47,200	37,600
Earnings (losses) before income taxes		360,100	1,260,800	162,500
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		5,237,300	4,945,700	4,451,000
Net premiums written		4,845,000	4,495,000	3,969,100
Net premiums earned		4,644,700	4,327,000	3,939,000
Net loss and LAE		3,386,900	2,961,100	2,869,300
Commissions, brokerage and other underwriting expenses	[1]	1,425,000	1,406,800	1,282,300
Underwriting (loss) profit	[2]	(167,200)	(40,900)	(212,600)
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		1,761,700	1,700,300	1,532,700
Net premiums written		1,439,600	1,328,800	1,169,400
Net premiums earned		1,379,700	1,280,100	1,177,300
Net loss and LAE		1,130,600	942,600	1,067,800
Commissions, brokerage and other underwriting expenses	[1]	424,200	424,200	384,900
Underwriting (loss) profit	[2]	(175,100)	(86,700)	(275,400)
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[3]	3,475,600	3,245,400	2,918,300
Net premiums written	[3]	3,405,400	3,166,200	2,799,700
Net premiums earned	[3]	3,265,000	3,046,900	2,761,700
Net loss and LAE	[3]	2,256,300	2,018,500	1,801,500
Commissions, brokerage and other underwriting expenses	[1],[3]	1,000,800	982,600	897,400
Underwriting (loss) profit	[2],[3]	7,900	45,800	62,800
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		2,125,700	1,738,400	1,470,800
Net premiums written		1,499,400	1,256,700	1,079,300
Net premiums earned		1,355,500	1,151,100	1,037,200
Net loss and LAE		952,200	725,300	651,100
Commissions, brokerage and other underwriting expenses	[1]	364,800	351,900	335,000
Underwriting (loss) profit	[2]	38,500	73,900	51,100
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		1,714,400	1,366,600	1,142,000
Net premiums written		1,124,800	912,000	773,300
Net premiums earned		1,008,800	824,200	747,300
Net loss and LAE		738,400	503,700	491,200
Commissions, brokerage and other underwriting expenses	[1]	226,100	219,200	211,900
Underwriting (loss) profit	[2]	44,300	101,300	44,200
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		411,300	371,800	328,800
Net premiums written		374,600	344,700	306,000
Net premiums earned		346,700	326,900	289,900
Net loss and LAE		213,800	221,600	159,900
Commissions, brokerage and other underwriting expenses	[1]	138,700	132,700	123,100
Underwriting (loss) profit	[2]	(5,800)	(27,400)	6,900
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		7,363,000	6,684,100	5,921,800
Net premiums written		6,344,400	5,751,700	5,048,400
Net premiums earned		6,000,200	5,478,100	4,976,200
Net loss and LAE		4,339,100	3,686,400	3,520,400
Commissions, brokerage and other underwriting expenses	[1]	1,789,800	1,758,700	1,617,300
Underwriting (loss) profit	[2]	(128,700)	33,000	(161,500)
Net investment income		465,700	533,200	480,900
Change in the fair value of equity securities		(55,800)	705,800	(184,200)
Net realized capital gains		37,100	6,000	31,600
Change in allowance for credit losses on available for sale securities		(8,000)
Change in allowance for credit losses on available for sale securities			(19,700)	(1,300)
Noninsurance revenue		34,900	27,000	22,000
Other operating expenses		103,600	103,100	59,800
Corporate administration		(300)	4,100	(1,700)
Amortization of intangible assets		800	1,300	(400)
Interest expense		27,000	27,100	27,100
Earnings (losses) before income taxes		214,100	1,149,700	102,700
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Gross premiums written	[4]	0	0	0
Net premiums written	[4]	0	0	0
Net premiums earned	[4]	0	0	0
Net loss and LAE	[4]	0	0	0
Commissions, brokerage and other underwriting expenses	[1],[4]	0	0	0
Underwriting (loss) profit	[2],[4]	0	0	0
Net investment income	[4]	1,900	6,300	4,800
Change in the fair value of equity securities	[4]	0	0	0
Net realized capital gains	[4]	35,500	1,000	900
Change in allowance for credit losses on available for sale securities	[4]	0
Change in allowance for credit losses on available for sale securities	[4]		0	0
Noninsurance revenue	[4]	2,477,500	2,289,300	1,574,600
Other operating expenses	[4]	2,310,000	2,132,900	1,485,600
Corporate administration	[4]	0	0	0
Amortization of intangible assets	[4]	43,400	32,500	24,400
Interest expense	[4]	15,500	20,100	10,500
Earnings (losses) before income taxes	[4]	146,000	111,100	59,800
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written		(33,600)	(27,700)	(26,200)
Net premiums written		0	0	0
Net premiums earned		0	0	0
Net loss and LAE		0	0	0
Commissions, brokerage and other underwriting expenses	[1]	0	0	0
Underwriting (loss) profit	[2]	0	0	0
Net investment income		23,300	10,700	14,800
Change in the fair value of equity securities		(54,700)	3,900	(44,800)
Net realized capital gains		(69,500)	(13,500)	(35,700)
Change in allowance for credit losses on available for sale securities		0
Change in allowance for credit losses on available for sale securities			0	0
Noninsurance revenue		8,700	12,500	47,400
Other operating expenses		15,700	27,300	33,900
Corporate administration		48,900	70,700	17,600
Amortization of intangible assets		0	0	0
Interest expense		45,700	52,800	53,100
Earnings (losses) before income taxes		$ (202,500)	$ (137,200)	$ (122,900)

[1] Includes amortization associated with deferred acquisition costs of $1,457.0 million, $1,392.8 million and $1,271.4 million for the years ended December 31, 2020, 2019 and 2018, respectively.

[2] 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.

[3] 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 & health; mortgage reinsurance; surety; and credit.

[4] Excludes certain minor, legacy investments that were previously reflected in Alleghany Capital in 2018 and prior periods, to align with management’s view of reportable segments.