Goodwill and Intangible Asset, Net of Accumulated Amortization Expense on Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross		$ 523,000	$ 455,100
Goodwill	[1]	523,021	455,142
Finite lived intangible assets, accumulated amortization		404,200	369,900
Intangible assets excluding goodwill, gross		1,090,200	923,000
Goodwill and other intangible assets, accumulated amortization		404,200	369,900
Intangible assets excluding goodwill, net of amortization	[1]	685,953	553,136
Goodwill and other intangible assets, Gross carrying value		1,613,200	1,378,100
Goodwill and other intangible assets, Net carrying value	[1]	1,209,000	1,008,200
Insurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross	[2]	49,000	49,000
Goodwill	[1],[2]	49,000	49,000
Finite lived intangible assets, accumulated amortization		74,800	73,400
Intangible assets excluding goodwill, gross		140,200	142,000
Goodwill and other intangible assets, accumulated amortization		74,800	73,400
Intangible assets excluding goodwill, net of amortization	[1]	65,400	68,600
Goodwill and other intangible assets, Gross carrying value		189,200	191,000
Goodwill and other intangible assets, Net carrying value	[1]	114,400	117,600
Insurance Segment | State insurance licenses
Goodwill and Intangible Assets Disclosure [Line Items]
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, 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		16,600	18,400
Finite lived intangible assets, accumulated amortization		12,400	11,400
Finite lived intangible assets, net	[1]	4,200	7,000
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
Insurance Segment | Renewal rights
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		25,100	25,100
Finite lived intangible assets, accumulated amortization		24,500	24,100
Finite lived intangible assets, net	[1]	600	1,000
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
Reinsurance Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross	[2]	8,800	8,800
Goodwill	[1],[2]	8,800	8,800
Finite lived intangible assets, accumulated amortization		225,700	225,300
Intangible assets excluding goodwill, gross		303,900	299,600
Goodwill and other intangible assets, accumulated amortization		225,700	225,300
Intangible assets excluding goodwill, net of amortization	[1]	78,200	74,300
Goodwill and other intangible assets, Gross carrying value		312,700	308,400
Goodwill and other intangible assets, Net carrying value	[1]	87,000	83,100
Reinsurance Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
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, 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	51,500
Finite lived intangible assets, accumulated amortization		30,700	26,000
Finite lived intangible assets, net	[1]	22,300	25,500
Reinsurance Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		15,100	14,600
Finite lived intangible assets, accumulated amortization		10,900	10,000
Finite lived intangible assets, net	[1]	4,200	4,600
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
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		(85,700)	(82,600)
Finite lived intangible assets, net	[1]	(13,100)	(16,200)
Reinsurance Segment | Leases
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross		(25,800)	(28,100)
Finite lived intangible assets, accumulated amortization		(21,600)	(19,500)
Finite lived intangible assets, net	[1]	(4,200)	(8,600)
Alleghany Capital Corporation Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill, gross	[2],[3]	465,200	397,300
Goodwill	[1],[2],[3]	465,200	397,300
Finite lived intangible assets, accumulated amortization	[3]	103,700	71,200
Intangible assets excluding goodwill, gross	[3]	646,100	481,400
Goodwill and other intangible assets, accumulated amortization	[3]	103,700	71,200
Intangible assets excluding goodwill, net of amortization	[1],[3]	542,400	410,200
Goodwill and other intangible assets, Gross carrying value	[3]	1,111,300	878,700
Goodwill and other intangible assets, Net carrying value	[1],[3]	1,007,600	807,500
Alleghany Capital Corporation Segment | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite Lived Intangible Assets Accumulated Amortization	[3]	600	100
Indefinite lived intangible assets, gross	[3]	171,800	176,900
Indefinite lived intangible assets, net	[1],[3]	171,200	176,800
Alleghany Capital Corporation Segment | Other
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	25,800	21,200
Finite lived intangible assets, accumulated amortization	[3]	23,000	20,800
Finite lived intangible assets, net	[1],[3]	2,800	400
Alleghany Capital Corporation Segment | License agreements
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	152,100	68,200
Finite lived intangible assets, accumulated amortization	[3]	32,600	22,600
Finite lived intangible assets, net	[1],[3]	119,500	45,600
Alleghany Capital Corporation Segment | Customer Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Finite lived intangible assets, gross	[3]	296,400	215,100
Finite lived intangible assets, accumulated amortization	[3]	47,500	27,700
Finite lived intangible assets, net	[1],[3]	$ 248,900	$ 187,400

[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 acquisition; and (vi) Kentucky Trailer on August 30, 2013 and its subsequent acquisitions.  Balances as of December 31, 2019 also reflect the finalization of provisional estimates that existed as of December 31, 2018.