Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross premiums written		$ 5,895,600	$ (23,300)	[1]	$ 5,767,100
Net premiums written		5,048,400	4,965,900		5,091,800
Net premiums earned		4,976,190	4,954,990		4,975,777
Net loss and LAE		3,520,431	3,620,197		2,917,166
Commissions, brokerage and other underwriting expenses		1,617,333	1,651,177		1,657,251
Underwriting (loss) profit	[2]	(161,500)	(316,400)		401,300	[3]
Net investment income		500,534	451,016		438,455
Change in the fair value of equity securities		(228,994)
Net realized capital gains		(3,241)	107,222		63,205
Other than temporary impairment losses		1,328
Other than temporary impairment losses			(16,871)		(45,165)
Other operating expenses		1,579,309	967,104		765,226
Corporate administration		15,732	46,998		42,960
Amortization of intangible assets		24,039	19,419		19,012
Interest expense		90,724	83,070		81,599
Earnings before income taxes		39,592	36,690		647,805
Corporate activities
Segment Reporting Information [Line Items]
Net realized capital gains	[1]		(1,500)
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		5,921,800	5,720,200		5,793,000
Net premiums written		5,048,400	4,965,900		5,091,800
Net premiums earned		4,976,200	4,955,000		4,975,800
Net loss and LAE		3,520,400	3,620,200		2,917,200
Commissions, brokerage and other underwriting expenses	[4]	1,617,300	1,651,200		1,657,300
Underwriting (loss) profit	[2]	(161,500)	(316,400)		401,300	[3]
Net investment income		484,000	437,300		430,800
Change in the fair value of equity securities		(184,200)
Net realized capital gains		32,500	108,700		172,700
Other than temporary impairment losses		(1,300)
Other than temporary impairment losses			(16,900)		(45,200)
Other operating expenses		1,545,400	927,700		723,700
Corporate administration		(1,700)	1,700		1,000
Amortization of intangible assets		24,000	19,400		19,000
Interest expense		37,600	30,900		29,100
Earnings before income taxes		160,800	145,400		869,000
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		4,451,000	4,210,600		4,330,300
Net premiums written		3,969,100	3,810,100		3,969,400
Net premiums earned		3,939,000	3,808,700		3,845,000
Net loss and LAE		2,869,300	2,785,400		2,285,400
Commissions, brokerage and other underwriting expenses	[4]	1,282,300	1,286,700		1,299,000
Underwriting (loss) profit	[2]	(212,600)	(263,400)		260,600	[3]
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		1,532,700	1,557,800		1,515,500
Net premiums written		1,169,400	1,233,100		1,237,200
Net premiums earned		1,177,300	1,181,900		1,168,000
Net loss and LAE		1,067,800	1,080,100		578,400
Commissions, brokerage and other underwriting expenses	[4]	384,900	383,400		376,200
Underwriting (loss) profit	[2]	(275,400)	(281,600)		213,400	[3]
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written		2,918,300	2,652,800	[3]	2,814,800	[3]
Net premiums written		2,799,700	2,577,000	[3]	2,732,200	[3]
Net premiums earned		2,761,700	2,626,800	[3]	2,677,000	[3]
Net loss and LAE		1,801,500	1,705,300	[3]	1,707,000	[3]
Commissions, brokerage and other underwriting expenses	[4]	897,400	903,300	[3]	922,800	[3]
Underwriting (loss) profit	[2]	62,800	18,200	[3]	47,200	[3]
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,470,800	1,509,600		1,462,700
Net premiums written		1,079,300	1,155,800		1,122,400
Net premiums earned		1,037,200	1,146,300		1,130,800
Net loss and LAE		651,100	834,800		631,800
Commissions, brokerage and other underwriting expenses	[4]	335,000	364,500		358,300
Underwriting (loss) profit	[2]	51,100	(53,000)		140,700	[3]
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		1,142,000	1,056,800		1,056,400
Net premiums written		773,300	724,400		734,100
Net premiums earned		747,300	721,700		754,500
Net loss and LAE		491,200	569,900		403,800
Commissions, brokerage and other underwriting expenses	[4]	211,900	208,900		212,300
Underwriting (loss) profit	[2]	44,200	(57,100)		138,400	[3]
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		328,800	290,200		266,500
Net premiums written		306,000	271,200		250,000
Net premiums earned		289,900	260,900		237,500
Net loss and LAE		159,900	143,900		125,300
Commissions, brokerage and other underwriting expenses	[4]	123,100	112,700		107,300
Underwriting (loss) profit	[2]	6,900	4,300		4,900	[3]
Operating Segments | Insurance Segment | Pacific Comp
Segment Reporting Information [Line Items]
Gross premiums written			162,600		139,800
Net premiums written			160,200		138,300
Net premiums earned			163,700		138,800
Net loss and LAE			121,000		102,700
Commissions, brokerage and other underwriting expenses	[4]		42,900		38,700
Underwriting (loss) profit	[2]		(200)		(2,600)	[3]
Operating Segments | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Net investment income		3,100	2,700		(2,300)
Net realized capital gains		900	23,000		12,800
Other operating expenses		1,485,600	844,900		643,100
Amortization of intangible assets		24,400	20,900		22,100
Interest expense		10,500	4,000		1,900
Earnings before income taxes		58,100	52,800		21,200
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		5,921,800	5,720,200		5,793,000
Net premiums written		5,048,400	4,965,900		5,091,800
Net premiums earned		4,976,200	4,955,000		4,975,800
Net loss and LAE		3,520,400	3,620,200		2,917,200
Commissions, brokerage and other underwriting expenses	[4]	1,617,300	1,651,200		1,657,300
Underwriting (loss) profit	[2]	(161,500)	(316,400)		401,300	[3]
Net investment income		480,900	434,600		433,100
Change in the fair value of equity securities		(184,200)
Net realized capital gains		31,600	85,700		159,900
Other than temporary impairment losses		(1,300)
Other than temporary impairment losses			(16,900)		(45,200)
Other operating expenses		59,800	82,800		80,600
Corporate administration		(1,700)	1,700		1,000
Amortization of intangible assets		(400)	(1,500)		(3,100)
Interest expense		27,100	26,900		27,200
Earnings before income taxes		102,700	92,600		847,800
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written	[1]	(26,200)	(23,300)		(25,900)
Net investment income	[1]	16,500	13,700		7,700
Change in the fair value of equity securities	[1]	(44,800)
Net realized capital gains	[1]	(35,700)	(1,500)		(109,500)
Other operating expenses	[1]	33,900	39,400		41,500
Corporate administration	[1]	17,600	45,300		42,000
Interest expense	[1]	53,100	52,100		52,500
Earnings before income taxes	[1]	(121,200)	(108,700)		(221,200)
Noninsurance Revenue
Segment Reporting Information [Line Items]
Noninsurance revenue		1,643,999	928,298		698,747
Noninsurance Revenue | Operating Segments
Segment Reporting Information [Line Items]
Noninsurance revenue		1,596,600	912,400		682,200
Noninsurance Revenue | Operating Segments | Alleghany Capital Corporation
Segment Reporting Information [Line Items]
Noninsurance revenue		1,574,600	896,900		677,800
Noninsurance Revenue | Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Noninsurance revenue		22,000	15,500		4,400
Noninsurance Revenue | Corporate activities
Segment Reporting Information [Line Items]
Noninsurance revenue	[1]	$ 47,400	$ 15,900		$ 16,600

[1] Includes elimination of minor reinsurance activity between segments.

[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, OTTI losses, 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 assumed 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.

[4] Includes amortization associated with deferred acquisition costs of $1,271.4 million, $1,261.7 million and $1,253.2 million for the years ended December 31, 2018, 2017 and 2016, respectively.