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Segments of Business - Additional Information (Detail) $ in Thousands		3 Months Ended
	Oct. 14, 2021 USD ($)	Mar. 31, 2022 USD ($) Segment	Dec. 31, 2021 USD ($)	Oct. 13, 2021
Segment Reporting Information [Line Items]
Number of reportable segments \| Segment		3
Senior notes and other debt		$ 2,822,777	$ 2,847,199
Goodwill		$ 754,648	753,607
Integrated Project Services L L C [Member]
Segment Reporting Information [Line Items]
Ownership of interest held by noncontrolling interests	18.00%	18.00%		15.00%
Anchorbuoy Limited Acquisition By Integrated Project Services Llc [Member]
Segment Reporting Information [Line Items]
Purchase price for acquisition	$ 262,500
Cash consideration paid for acquisition	98,600
Incremental debt	125,100
Goodwill	102,100
Issuance of Non controlling interest	38,800
Anchorbuoy Limited Acquisition By Integrated Project Services Llc [Member] \| Trade Names and Trade Marks [Member]
Segment Reporting Information [Line Items]
Indefinite lived intangible assets	15,500
Anchorbuoy Limited Acquisition By Integrated Project Services Llc [Member] \| Alleghany Corporation
Segment Reporting Information [Line Items]
Contribution from Alleghany	97,400
Anchorbuoy Limited Acquisition By Integrated Project Services Llc [Member] \| Customer Relationships
Segment Reporting Information [Line Items]
Finite lived intangible assets	$ 124,700
Operating Segments \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Senior notes and other debt		$ 756,000	$ 780,500
Operating Segments \| Alleghany Capital Corporation Segment \| Integrated Project Services L L C [Member]
Segment Reporting Information [Line Items]
Senior notes and other debt		172,100
Operating Segments \| Alleghany Capital Corporation Segment \| Jazwares, LLC
Segment Reporting Information [Line Items]
Senior notes and other debt		267,900
Operating Segments \| Alleghany Capital Corporation Segment \| Kentucky Trailer
Segment Reporting Information [Line Items]
Senior notes and other debt		60,400
Operating Segments \| Alleghany Capital Corporation Segment \| Wilbert Funeral Services, Inc
Segment Reporting Information [Line Items]
Senior notes and other debt		74,700
Operating Segments \| Alleghany Capital Corporation Segment \| WWSC Holdings, LLC
Segment Reporting Information [Line Items]
Senior notes and other debt		125,400
Operating Segments \| Alleghany Capital Corporation Segment \| Piedmont Manufacturing Group, LLC
Segment Reporting Information [Line Items]
Senior notes and other debt		31,400
Operating Segments \| Alleghany Capital Corporation Segment \| Precision Cutting Technologies, Inc
Segment Reporting Information [Line Items]
Senior notes and other debt		24,100
Operating Segments \| Alleghany Capital Corporation Segment \| Concord \| Floating Rate Debt Funded by Alleghany Corporation
Segment Reporting Information [Line Items]
Senior notes and other debt		$ 33,000