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Segments of Business - Additional Information (Detail) $ in Thousands			9 Months Ended
	Oct. 01, 2018 USD ($)	Feb. 07, 2018 USD ($)	Sep. 30, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments \| Segment			3
Debt amount			$ 1,695,303	$ 1,669,039
Wilbert Funeral Services, Inc
Segment Reporting Information [Line Items]
Equity interest percentage			45.00%
Operating Segments \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Debt amount			$ 311,300	$ 284,500
Operating Segments \| Alleghany Capital Corporation Segment \| Concord
Segment Reporting Information [Line Items]
Business acquisition date	Oct. 01, 2018
Ownership interest acquired	85.00%
Purchase price for acquisition	$ 136,600
Cash consideration paid for acquisition	68,600
Potential contingent consideration on acquisition	38,200
Incremental debt acquired	29,800
Goodwill	83,000
Finite-lived intangible assets acquired	$ 70,800
Operating Segments \| Alleghany Capital Corporation Segment \| WWSC Holdings, LLC
Segment Reporting Information [Line Items]
Debt amount			80,400
Operating Segments \| Alleghany Capital Corporation Segment \| WWSC Holdings, LLC \| Hirschfeld Holdings LP
Segment Reporting Information [Line Items]
Business acquisition date		Feb. 07, 2018
Purchase price for acquisition		$ 109,100
Cash consideration paid for acquisition		94,400
Incremental debt acquired		14,700
Goodwill		3,000
Finite-lived intangible assets acquired		9,400
Capital contributions from Alleghany		75,500
Capital contributions from noncontrolling interests		$ 18,900
Operating Segments \| Alleghany Capital Corporation Segment \| Jazwares, LLC
Segment Reporting Information [Line Items]
Debt amount			48,900
Operating Segments \| Alleghany Capital Corporation Segment \| Kentucky Trailer
Segment Reporting Information [Line Items]
Debt amount			74,600
Operating Segments \| Alleghany Capital Corporation Segment \| Integrated Project Services LLC
Segment Reporting Information [Line Items]
Debt amount			41,900
Operating Segments \| Alleghany Capital Corporation Segment \| Concord
Segment Reporting Information [Line Items]
Debt amount			32,600
Operating Segments \| Alleghany Capital Corporation Segment \| Precision Cutting Technologies, Inc
Segment Reporting Information [Line Items]
Debt amount			$ 32,900