Goodwill and Intangible Assets - Summary of Rollforward of Goodwill and Intangible Assets by Segment (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021		Dec. 31, 2020
Goodwill [Line Items]
Goodwill, beginning balance		$ 614,163	[1]	$ 523,000
Acquired	[2]	139,400		92,100
Other increase (decrease)		(0)		(900)
Goodwill, ending balance	[1]	753,607		614,163
Intangible assets, beginning balance		787,462	[1]	686,000
Acquired	[2]	189,400		147,000
Amortization	[3]	(49,900)		(44,200)
Other increase (decrease)		(2,600)		(1,300)
Intangible assets, ending balance	[1]	924,406		787,462
Alleghany Capital Segment
Goodwill [Line Items]
Goodwill, beginning balance		557,300	[1],[4],[5]	465,200
Acquired	[2]	139,400		92,100
Other increase (decrease)		0		0
Goodwill, ending balance	[1],[4],[5]	696,700		557,300
Intangible assets, beginning balance		646,000	[1],[4]	542,400
Acquired	[2]	189,400		147,000
Amortization	[3]	(48,100)		(43,400)
Other increase (decrease)		0		0
Intangible assets, ending balance	[1],[4]	787,300		646,000
Insurance Segment
Goodwill [Line Items]
Goodwill, beginning balance		48,100	[1],[5]	49,000
Acquired	[2]	0		0
Other increase (decrease)		0		(900)
Goodwill, ending balance	[1],[5]	48,100		48,100
Intangible assets, beginning balance		63,100	[1]	65,400
Acquired	[2]	0		0
Amortization	[3]	(700)		(1,000)
Other increase (decrease)		(1,300)		(1,300)
Intangible assets, ending balance	[1]	61,100		63,100
Reinsurance Segment
Goodwill [Line Items]
Goodwill, beginning balance		8,800	[1],[5]	8,800
Acquired	[2]	0		0
Other increase (decrease)		0		0
Goodwill, ending balance	[1],[5]	8,800		8,800
Intangible assets, beginning balance		78,400	[1]	78,200
Acquired	[2]	0		0
Amortization	[3]	(1,100)		200
Other increase (decrease)		(1,300)		0
Intangible assets, ending balance	[1]	$ 76,000		$ 78,400

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

[2] See description below for a summary of recent material acquisitions impacting goodwill and intangible assets.

[3] See table below for additional information regarding amortization and useful lives by major class of intangible asset.

[4] 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.

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