Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions						12 Months Ended
		Apr. 01, 2020	Oct. 01, 2019	Oct. 01, 2018	Feb. 07, 2018	Dec. 31, 2020	Dec. 31, 2019	Mar. 31, 2020	Sep. 30, 2019
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill	[1]					$ 92.1	$ 67.9
Alleghany Capital Corporation Segment
Goodwill and Intangible Assets Disclosure [Line Items]
Goodwill	[1]					$ 92.1	$ 67.9
Jazwares, LLC
Goodwill and Intangible Assets Disclosure [Line Items]
Ownership of interest held by noncontrolling partners						24.00%
License Agreements
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						8 years
Agency Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						15 years
Brokerage and Reinsurance Relationships
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						15 years
Renewal rights | Minimum
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						3 years
Renewal rights | Maximum
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						14 years
Loss and LAE reserves
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						15 years
Leases
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						10 years
Customer Relationships | Minimum
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						5 years
Customer Relationships | Maximum
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						10 years
Wilbert Funeral Services, Inc
Goodwill and Intangible Assets Disclosure [Line Items]
Ownership interest acquired		55.00%
Equity interest percentage		100.00%
Purchase price for acquisition		$ 121.3
Cash consideration paid for acquisition		46.3
Incremental debt acquired		75.0
Goodwill		62.9
Gain on remeasurement of pre-existing equity ownership to estimated fair value		16.3
Equity interest percentage								45.00%
Wilbert Funeral Services, Inc | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite-lived intangible assets acquired		26.2
Wilbert Funeral Services, Inc | Customer Relationship
Goodwill and Intangible Assets Disclosure [Line Items]
Finite-lived intangible assets acquired		21.5
Wilbert Funeral Services, Inc | License Agreements
Goodwill and Intangible Assets Disclosure [Line Items]
Finite-lived intangible assets acquired		$ 14.9
Wicked Cool, Toys LLC | Jazwares, LLC | Alleghany Capital Corporation Segment | Operating Segments
Goodwill and Intangible Assets Disclosure [Line Items]
Ownership of interest held by noncontrolling partners			25.00%						23.00%
Wicked Cool, Toys LLC | Jazwares, LLC
Goodwill and Intangible Assets Disclosure [Line Items]
Purchase price for acquisition			$ 159.0
Incremental debt acquired			135.5
Goodwill			39.1
Issuance of certain noncontrolling interests in Jazwares			23.5
Wicked Cool, Toys LLC | License Agreements | Jazwares, LLC
Goodwill and Intangible Assets Disclosure [Line Items]
Finite-lived intangible assets acquired			83.9
Wicked Cool, Toys LLC | Finite-Lived Intangible Assets Other Than License Agreements | Jazwares, LLC
Goodwill and Intangible Assets Disclosure [Line Items]
Finite-lived intangible assets acquired			$ 24.9
Concord | Alleghany Capital Corporation Segment | Operating Segments
Goodwill and Intangible Assets Disclosure [Line Items]
Ownership interest acquired				85.00%
Purchase price for acquisition				$ 136.6
Cash consideration paid for acquisition				68.6
Incremental debt acquired				29.8
Goodwill				83.0
Finite-lived intangible assets acquired				70.8
Potential contingent consideration on acquisition				$ 38.2
Business acquisition date				Oct. 01, 2018
Hirschfeld Holdings LP | WWSC Holdings, LLC | Alleghany Capital Corporation Segment | Operating Segments
Goodwill and Intangible Assets Disclosure [Line Items]
Purchase price for acquisition					$ 109.1
Cash consideration paid for acquisition					94.4
Incremental debt acquired					14.7
Goodwill					3.0
Finite-lived intangible assets acquired					$ 9.4
Business acquisition date					Feb. 07, 2018
Capital contributions from Alleghany					$ 75.5
Capital contributions from noncontrolling interests					$ 18.9

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