Results for Reportable Segments and Corporate Activities (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Gross premiums written		$ 6,656,400	$ 5,895,600	$ 5,696,900
Net premiums written		5,751,700	5,048,400	4,965,900
Net premiums earned		5,478,143	4,976,190	4,954,990
Net loss and LAE		3,686,435	3,520,431	3,620,197
Commissions, brokerage and other underwriting expenses	[1]	1,758,698	1,617,333	1,651,177
Underwriting (loss) profit	[2]	33,000	(161,500)	(316,400)
Net investment income		550,241	500,534	451,016
Change in the fair value of equity securities		709,695	(228,994)
Net realized capital gains		(6,551)	(3,241)	107,222
Other than temporary impairment losses		(19,660)	(1,328)
Other than temporary impairment losses				(16,871)
Noninsurance revenue		2,328,848	1,643,999	928,298
Other operating expenses		2,263,326	1,579,309	967,104
Corporate administration		74,830	15,732	46,998
Amortization of intangible assets		33,834	24,039	19,419
Interest expense		99,957	90,724	83,070
Earnings (losses) before income taxes		1,123,636	39,592	36,690
Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written		6,684,100	5,921,800	5,720,200
Net premiums written		5,751,700	5,048,400	4,965,900
Net premiums earned		5,478,100	4,976,200	4,955,000
Net loss and LAE		3,686,400	3,520,400	3,620,200
Commissions, brokerage and other underwriting expenses	[1]	1,758,700	1,617,300	1,651,200
Underwriting (loss) profit	[2]	33,000	(161,500)	(316,400)
Net investment income		539,500	485,700	436,700
Change in the fair value of equity securities		705,800	(184,200)
Net realized capital gains		7,000	32,500	108,700
Other than temporary impairment losses		(19,700)	(1,300)
Other than temporary impairment losses				(16,900)
Noninsurance revenue		2,316,300	1,596,600	912,400
Other operating expenses		2,236,000	1,545,400	927,700
Corporate administration		4,100	(1,700)	1,700
Amortization of intangible assets		33,800	24,000	19,400
Interest expense		47,200	37,600	30,900
Earnings (losses) before income taxes		1,260,800	162,500	144,800
Operating Segments | Reinsurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		4,945,700	4,451,000	4,210,600
Net premiums written		4,495,000	3,969,100	3,810,100
Net premiums earned		4,327,000	3,939,000	3,808,700
Net loss and LAE		2,961,100	2,869,300	2,785,400
Commissions, brokerage and other underwriting expenses	[1]	1,406,800	1,282,300	1,286,700
Underwriting (loss) profit	[2]	(40,900)	(212,600)	(263,400)
Operating Segments | Reinsurance Segment | Property
Segment Reporting Information [Line Items]
Gross premiums written		1,700,300	1,532,700	1,557,800
Net premiums written		1,328,800	1,169,400	1,233,100
Net premiums earned		1,280,100	1,177,300	1,181,900
Net loss and LAE		942,600	1,067,800	1,080,100
Commissions, brokerage and other underwriting expenses	[1]	424,200	384,900	383,400
Underwriting (loss) profit	[2]	(86,700)	(275,400)	(281,600)
Operating Segments | Reinsurance Segment | Casualty & Other
Segment Reporting Information [Line Items]
Gross premiums written	[3]	3,245,400	2,918,300	2,652,800
Net premiums written	[3]	3,166,200	2,799,700	2,577,000
Net premiums earned	[3]	3,046,900	2,761,700	2,626,800
Net loss and LAE	[3]	2,018,500	1,801,500	1,705,300
Commissions, brokerage and other underwriting expenses	[1],[3]	982,600	897,400	903,300
Underwriting (loss) profit	[2],[3]	45,800	62,800	18,200
Operating Segments | Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		1,738,400	1,470,800	1,509,600
Net premiums written		1,256,700	1,079,300	1,155,800
Net premiums earned		1,151,100	1,037,200	1,146,300
Net loss and LAE		725,300	651,100	834,800
Commissions, brokerage and other underwriting expenses	[1]	351,900	335,000	364,500
Underwriting (loss) profit	[2]	73,900	51,100	(53,000)
Operating Segments | Insurance Segment | RSUI
Segment Reporting Information [Line Items]
Gross premiums written		1,366,600	1,142,000	1,056,800
Net premiums written		912,000	773,300	724,400
Net premiums earned		824,200	747,300	721,700
Net loss and LAE		503,700	491,200	569,900
Commissions, brokerage and other underwriting expenses	[1]	219,200	211,900	208,900
Underwriting (loss) profit	[2]	101,300	44,200	(57,100)
Operating Segments | Insurance Segment | CapSpecialty Incorporated
Segment Reporting Information [Line Items]
Gross premiums written		371,800	328,800	290,200
Net premiums written		344,700	306,000	271,200
Net premiums earned		326,900	289,900	260,900
Net loss and LAE		221,600	159,900	143,900
Commissions, brokerage and other underwriting expenses	[1]	132,700	123,100	112,700
Underwriting (loss) profit	[2]	(27,400)	6,900	4,300
Operating Segments | Insurance Segment | Pacific Comp
Segment Reporting Information [Line Items]
Gross premiums written				162,600
Net premiums written				160,200
Net premiums earned				163,700
Net loss and LAE				121,000
Commissions, brokerage and other underwriting expenses	[1]			42,900
Underwriting (loss) profit	[2]			(200)
Operating Segments | Reinsurance Segment and Insurance Segment
Segment Reporting Information [Line Items]
Gross premiums written		6,684,100	5,921,800	5,720,200
Net premiums written		5,751,700	5,048,400	4,965,900
Net premiums earned		5,478,100	4,976,200	4,955,000
Net loss and LAE		3,686,400	3,520,400	3,620,200
Commissions, brokerage and other underwriting expenses	[1]	1,758,700	1,617,300	1,651,200
Underwriting (loss) profit	[2]	33,000	(161,500)	(316,400)
Net investment income		533,200	480,900	434,600
Change in the fair value of equity securities		705,800	(184,200)
Net realized capital gains		6,000	31,600	85,700
Other than temporary impairment losses		(19,700)	(1,300)
Other than temporary impairment losses				(16,900)
Noninsurance revenue		27,000	22,000	15,500
Other operating expenses		103,100	59,800	82,800
Corporate administration		4,100	(1,700)	1,700
Amortization of intangible assets		1,300	(400)	(1,500)
Interest expense		27,100	27,100	26,900
Earnings (losses) before income taxes		1,149,700	102,700	92,600
Operating Segments | Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Net investment income	[4]	6,300	4,800	2,100
Net realized capital gains	[4]	1,000	900	23,000
Noninsurance revenue	[4]	2,289,300	1,574,600	896,900
Other operating expenses	[4]	2,132,900	1,485,600	844,900
Amortization of intangible assets	[4]	32,500	24,400	20,900
Interest expense	[4]	20,100	10,500	4,000
Earnings (losses) before income taxes	[4]	111,100	59,800	52,200
Corporate activities
Segment Reporting Information [Line Items]
Gross premiums written	[5]	(27,700)	(26,200)	(23,300)
Net investment income	[5]	10,700	14,800	14,300
Change in the fair value of equity securities	[5]	3,900	(44,800)
Net realized capital gains	[5]	(13,500)	(35,700)	(1,500)
Noninsurance revenue	[5]	12,500	47,400	15,900
Other operating expenses	[5]	27,300	33,900	39,400
Corporate administration	[5]	70,700	17,600	45,300
Interest expense	[5]	52,800	53,100	52,100
Earnings (losses) before income taxes	[5]	$ (137,200)	$ (122,900)	$ (108,100)

[1] Includes amortization associated with deferred acquisition costs of $1,392.8 million, $1,271.4 million and $1,261.7 million for the years ended December 31, 2019, 2018 and 2017, 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, 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 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.

[5] Includes elimination of minor reinsurance activity between segments. Also, includes certain minor, legacy investments that were previously reflected in Alleghany Capital in 2018 and prior periods.