Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions						12 Months Ended
	Oct. 01, 2019	Oct. 01, 2018	Feb. 07, 2018	May 01, 2017	Apr. 28, 2017	Dec. 31, 2019	Sep. 30, 2019
License agreements
Goodwill and Intangible Assets Disclosure [Line Items]
Economic useful lives of significant intangible assets						8 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
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
Jazwares, LLC
Goodwill and Intangible Assets Disclosure [Line Items]
Ownership of interest held by noncontrolling partners	25.00%						23.00%
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
Issuance of certain noncontrolling interests in Jazwares	23.5
Goodwill	39.1
Wicked Cool, Toys LLC | Jazwares, LLC | License agreements
Goodwill and Intangible Assets Disclosure [Line Items]
Finite-lived intangible assets acquired	83.9
Wicked Cool, Toys LLC | Jazwares, LLC | Finite-Lived Intangible Assets Other Than License Agreements
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]
Purchase price for acquisition		$ 136.6
Incremental debt acquired		29.8
Goodwill		83.0
Finite-lived intangible assets acquired		$ 70.8
Ownership interest acquired		85.00%
Cash consideration paid for acquisition		$ 68.6
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
Incremental debt acquired			14.7
Goodwill			3.0
Finite-lived intangible assets acquired			9.4
Cash consideration paid for acquisition			$ 94.4
Business acquisition date			Feb. 07, 2018
Capital contributions from Alleghany			$ 75.5
Capital contributions from noncontrolling interests			$ 18.9
WWSC Holdings, LLC | Alleghany Capital Corporation Segment | Operating Segments
Goodwill and Intangible Assets Disclosure [Line Items]
Purchase price for acquisition					$ 164.5
Goodwill					$ 41.1
Ownership interest acquired					80.00%
Cash consideration paid for acquisition				$ 163.9
Business acquisition date					Apr. 28, 2017
Purchase price adjustments on acquisition					$ 0.6
WWSC Holdings, LLC | Alleghany Capital Corporation Segment | Operating Segments | Trade name
Goodwill and Intangible Assets Disclosure [Line Items]
Indefinite-lived intangible assets acquired					25.3
WWSC Holdings, LLC | Customer Relationships | Alleghany Capital Corporation Segment | Operating Segments
Goodwill and Intangible Assets Disclosure [Line Items]
Finite-lived intangible assets acquired					$ 70.0