Goodwill and Intangible Asset, Net of Accumulated Amortization Expense on Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands		Dec. 31, 2021		Dec. 31, 2020		Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross		$ 753,600		$ 614,200
Accumulated Amortization		0		0
Goodwill		753,607	[1]	614,163	[1]	$ 523,000
Finite lived intangible assets, accumulated amortization		496,700		446,700
Intangible assets excluding goodwill, gross		1,421,100		1,234,200
Goodwill and other intangible assets, accumulated amortization		496,700		446,700
Intangible assets excluding goodwill, net of amortization		924,406	[1]	787,462	[1]	686,000
Goodwill and other intangible assets, Gross carrying value		2,174,700		1,848,400
Goodwill and other intangible assets, Net carrying value	[1]	1,678,000		1,401,700
Insurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross	[2]	48,100		48,100
Accumulated Amortization	[2]	0		0
Goodwill		48,100	[1],[2]	48,100	[1],[2]	49,000
Finite lived intangible assets, accumulated amortization		74,800		74,100
Intangible assets excluding goodwill, gross		135,900		137,200
Goodwill and other intangible assets, accumulated amortization		74,800		74,100
Intangible assets excluding goodwill, net of amortization		61,100	[1]	63,100	[1]	65,400
Goodwill and other intangible assets, Gross carrying value		184,000		185,300
Goodwill and other intangible assets, Net carrying value	[1]	109,200		111,200
Insurance Segment | State insurance licenses
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite Lived Intangible Assets Accumulated Amortization		0		0
Indefinite lived intangible assets, gross		25,100		25,100
Indefinite lived intangible assets, net	[1]	25,100		25,100
Insurance Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite Lived Intangible Assets Accumulated Amortization		0		0
Indefinite lived intangible assets, gross		35,500		35,500
Indefinite lived intangible assets, net	[1]	35,500		35,500
Insurance Segment | Agency Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		12,500		13,800
Finite lived intangible assets, accumulated amortization		12,100		11,600
Finite lived intangible assets, net	[1]	400		2,200
Insurance Segment | Brokerage and Reinsurance Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		33,800		33,800
Finite lived intangible assets, accumulated amortization		33,800		33,800
Finite lived intangible assets, net	[1]	0		0
Insurance Segment | Renewal rights
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		24,900		24,900
Finite lived intangible assets, accumulated amortization		24,800		24,600
Finite lived intangible assets, net	[1]	100		300
Insurance Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		4,100		4,100
Finite lived intangible assets, accumulated amortization		4,100		4,100
Finite lived intangible assets, net	[1]	0		0
Reinsurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross	[2]	8,800		8,800
Accumulated Amortization	[2]	0		0
Goodwill		8,800	[1],[2]	8,800	[1],[2]	8,800
Finite lived intangible assets, accumulated amortization		226,700		225,500
Intangible assets excluding goodwill, gross		302,700		303,900
Goodwill and other intangible assets, accumulated amortization		226,700		225,500
Intangible assets excluding goodwill, net of amortization		76,000	[1]	78,400	[1]	78,200
Goodwill and other intangible assets, Gross carrying value		311,500		312,700
Goodwill and other intangible assets, Net carrying value	[1]	84,800		87,200
Reinsurance Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite Lived Intangible Assets Accumulated Amortization		0		0
Indefinite lived intangible assets, gross		50,000		50,000
Indefinite lived intangible assets, net	[1]	50,000		50,000
Reinsurance Segment | State and Foreign Insurance Licenses
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite Lived Intangible Assets Accumulated Amortization		0		0
Indefinite lived intangible assets, gross		19,000		19,000
Indefinite lived intangible assets, net	[1]	19,000		19,000
Reinsurance Segment | Renewal rights
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		51,800		53,000
Finite lived intangible assets, accumulated amortization		40,400		35,200
Finite lived intangible assets, net	[1]	11,400		17,800
Reinsurance Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		15,100		15,100
Finite lived intangible assets, accumulated amortization		11,600		11,200
Finite lived intangible assets, net	[1]	3,500		3,900
Reinsurance Segment | Value of business in-force
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		291,400		291,400
Finite lived intangible assets, accumulated amortization		291,400		291,400
Finite lived intangible assets, net	[1]	0		0
Reinsurance Segment | Loss and LAE reserves
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		(98,800)		(98,800)
Finite lived intangible assets, accumulated amortization		(90,900)		(88,600)
Finite lived intangible assets, net	[1]	(7,900)		(10,200)
Reinsurance Segment | Leases
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		(25,800)		(25,800)
Finite lived intangible assets, accumulated amortization		(25,800)		(23,700)
Finite lived intangible assets, net	[1]	0		(2,100)
Alleghany Capital Corporation Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross	[2],[3]	696,700		557,300
Accumulated Amortization	[2],[3]	0		0
Goodwill		696,700	[1],[2],[3]	557,300	[1],[2],[3]	465,200
Finite lived intangible assets, accumulated amortization	[3]	195,200		147,100
Intangible assets excluding goodwill, gross	[3]	982,500		793,100
Goodwill and other intangible assets, accumulated amortization	[3]	195,200		147,100
Intangible assets excluding goodwill, net of amortization		787,300	[1],[3]	646,000	[1],[3]	$ 542,400
Goodwill and other intangible assets, Gross carrying value	[3]	1,679,200		1,350,400
Goodwill and other intangible assets, Net carrying value	[1],[3]	1,484,000		1,203,300
Alleghany Capital Corporation Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite Lived Intangible Assets Accumulated Amortization	[3]	5,200		2,700
Indefinite lived intangible assets, gross	[3]	245,300		218,200
Indefinite lived intangible assets, net	[1],[3]	240,100		215,500
Alleghany Capital Corporation Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	50,000		50,000
Finite lived intangible assets, accumulated amortization	[3]	25,800		23,900
Finite lived intangible assets, net	[1],[3]	24,200		26,100
Alleghany Capital Corporation Segment | License Agreements
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	167,100		167,100
Finite lived intangible assets, accumulated amortization	[3]	63,300		48,100
Finite lived intangible assets, net	[1],[3]	103,800		119,000
Alleghany Capital Corporation Segment | Customer Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	520,100		357,800
Finite lived intangible assets, accumulated amortization	[3]	100,900		72,400
Finite lived intangible assets, net	[1],[3]	$ 419,200		$ 285,400

[1] Goodwill and intangible assets have been reduced by amounts written-down in prior periods, as applicable.

[2] See Note 13 for additional information on Alleghany’s segments of business.

[3] Represents goodwill and other intangible assets related to the acquisition of: (i) Jazwares on April 15, 2016 and its subsequent acquisitions; (ii) W&W|AFCO Steel on April 28, 2017 and its subsequent acquisition; (iii) Concord on October 1, 2018; (iv) PCT on April 26, 2012 and its subsequent acquisitions; (v) IPS on October 31, 2015 and its subsequent acquisitions; (vi) Kentucky Trailer on August 30, 2013 and its subsequent acquisitions; (vii) Wilbert on April 1, 2020 and its subsequent acquisitions; and (viii) WPS on May 10, 2021.