Goodwill and Intangible Assets - Summary of Rollforward of Goodwill and Intangible Assets by Segment (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019
Goodwill [Line Items]
Goodwill, beginning balance		$ 523,021	[1]	$ 455,100
Acquired	[2]	92,100		67,900
Other increase (decrease)		(900)		0
Goodwill, ending balance	[1]	614,163		523,021
Intangible assets, beginning balance		685,953	[1]	553,100
Acquired	[2]	147,000		166,200
Amortization	[3]	(44,200)		(33,800)
Other increase (decrease)		(1,300)		500
Intangible assets, ending balance	[1]	787,462		685,953
Insurance Segment
Goodwill [Line Items]
Goodwill, beginning balance		49,000	[1],[4]	49,000
Acquired	[2]	0		0
Other increase (decrease)		(900)		0
Goodwill, ending balance	[1],[4]	48,100		49,000
Intangible assets, beginning balance		65,400	[1]	68,600
Acquired	[2]	0		0
Amortization	[3]	(1,000)		(1,400)
Other increase (decrease)		(1,300)		(1,800)
Intangible assets, ending balance	[1]	63,100		65,400
Reinsurance Segment
Goodwill [Line Items]
Goodwill, beginning balance		8,800	[1],[4]	8,800
Acquired	[2]	0		0
Other increase (decrease)		0		0
Goodwill, ending balance	[1],[4]	8,800		8,800
Intangible assets, beginning balance		78,200	[1]	74,300
Acquired	[2]	0		1,500
Amortization	[3]	200		100
Other increase (decrease)		0		2,300
Intangible assets, ending balance	[1]	78,400		78,200
Alleghany Capital Segment
Goodwill [Line Items]
Goodwill, beginning balance		465,200	[1],[4],[5]	397,300
Acquired	[2]	92,100		67,900
Other increase (decrease)		0		0
Goodwill, ending balance	[1],[4],[5]	557,300		465,200
Intangible assets, beginning balance		542,400	[1],[5]	410,200
Acquired	[2]	147,000		164,700
Amortization	[3]	(43,400)		(32,500)
Other increase (decrease)		0		0
Intangible assets, ending balance	[1],[5]	$ 646,000		$ 542,400

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

[5] 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.