Goodwill and Intangible Asset, Net of Accumulated Amortization Expense on Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross		$ 614,200		$ 523,000
Goodwill		614,163	[1]	523,021	[1]	$ 455,100
Finite lived intangible assets, accumulated amortization		446,700		404,200
Intangible assets excluding goodwill, gross		1,234,200		1,090,200
Goodwill and other intangible assets, accumulated amortization		446,700		404,200
Intangible assets excluding goodwill, net of amortization		787,462	[1]	685,953	[1]	553,100
Goodwill and other intangible assets, Gross carrying value		1,848,400		1,613,200
Goodwill and other intangible assets, Net carrying value	[1]	1,401,700		1,209,000
Insurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross	[2]	48,100		49,000
Goodwill		48,100	[1],[2]	49,000	[1],[2]	49,000
Finite lived intangible assets, accumulated amortization		74,100		74,800
Intangible assets excluding goodwill, gross		137,200		140,200
Goodwill and other intangible assets, accumulated amortization		74,100		74,800
Intangible assets excluding goodwill, net of amortization		63,100	[1]	65,400	[1]	68,600
Goodwill and other intangible assets, Gross carrying value		185,300		189,200
Goodwill and other intangible assets, Net carrying value	[1]	111,200		114,400
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		13,800		16,600
Finite lived intangible assets, accumulated amortization		11,600		12,400
Finite lived intangible assets, net	[1]	2,200		4,200
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		25,100
Finite lived intangible assets, accumulated amortization		24,600		24,500
Finite lived intangible assets, net	[1]	300		600
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
Goodwill		8,800	[1],[2]	8,800	[1],[2]	8,800
Finite lived intangible assets, accumulated amortization		225,500		225,700
Intangible assets excluding goodwill, gross		303,900		303,900
Goodwill and other intangible assets, accumulated amortization		225,500		225,700
Intangible assets excluding goodwill, net of amortization		78,400	[1]	78,200	[1]	74,300
Goodwill and other intangible assets, Gross carrying value		312,700		312,700
Goodwill and other intangible assets, Net carrying value	[1]	87,200		87,000
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		53,000		53,000
Finite lived intangible assets, accumulated amortization		35,200		30,700
Finite lived intangible assets, net	[1]	17,800		22,300
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		11,200		10,900
Finite lived intangible assets, net	[1]	3,900		4,200
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		(88,600)		(85,700)
Finite lived intangible assets, net	[1]	(10,200)		(13,100)
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		(23,700)		(21,600)
Finite lived intangible assets, net	[1]	(2,100)		(4,200)
Alleghany Capital Corporation Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross	[2],[3]	557,300		465,200
Goodwill		557,300	[1],[2],[3]	465,200	[1],[2],[3]	397,300
Finite lived intangible assets, accumulated amortization	[3]	147,100		103,700
Intangible assets excluding goodwill, gross	[3]	793,100		646,100
Goodwill and other intangible assets, accumulated amortization	[3]	147,100		103,700
Intangible assets excluding goodwill, net of amortization		646,000	[1],[3]	542,400	[1],[3]	$ 410,200
Goodwill and other intangible assets, Gross carrying value	[3]	1,350,400		1,111,300
Goodwill and other intangible assets, Net carrying value	[1],[3]	1,203,300		1,007,600
Alleghany Capital Corporation Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite Lived Intangible Assets Accumulated Amortization	[3]	2,700		600
Indefinite lived intangible assets, gross	[3]	218,200		171,800
Indefinite lived intangible assets, net	[1],[3]	215,500		171,200
Alleghany Capital Corporation Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	50,000		25,800
Finite lived intangible assets, accumulated amortization	[3]	23,900		23,000
Finite lived intangible assets, net	[1],[3]	26,100		2,800
Alleghany Capital Corporation Segment | License Agreements
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	167,100		152,100
Finite lived intangible assets, accumulated amortization	[3]	48,100		32,600
Finite lived intangible assets, net	[1],[3]	119,000		119,500
Alleghany Capital Corporation Segment | Customer Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	357,800		296,400
Finite lived intangible assets, accumulated amortization	[3]	72,400		47,500
Finite lived intangible assets, net	[1],[3]	$ 285,400		$ 248,900

[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 acquisition; (vi) Kentucky Trailer on August 30, 2013 and its subsequent acquisitions; and (vii) Wilbert on April 1, 2020.  Balances as of December 31, 2020 also reflect the finalization of certain provisional estimates that existed as of December 31, 2019.