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Segments of Business - Additional Information (Detail) $ in Thousands		3 Months Ended		12 Months Ended
	Feb. 07, 2018 USD ($)	Mar. 31, 2018 USD ($) Segment	Mar. 31, 2017 Segment	Dec. 31, 2017 USD ($)	Aug. 01, 2017	Apr. 28, 2017
Segment Reporting Information [Line Items]
Number of reportable segments \| Segment		3	3
Debt amount		$ 1,540,865		$ 1,484,897
WWSC Holdings, LLC
Segment Reporting Information [Line Items]
Business acquisition date		Apr. 28, 2017
WWSC Holdings, LLC \| Hirschfeld Holdings, LP
Segment Reporting Information [Line Items]
Business acquisition date		Feb. 07, 2018
Wilbert Funeral Services, Inc
Segment Reporting Information [Line Items]
Equity interest percentage		45.00%			45.00%
Operating Segments \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Debt amount		$ 156,900
Operating Segments \| WWSC Holdings, LLC \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Business acquisition date		Apr. 28, 2017
Ownership interest acquired						80.00%
Operating Segments \| WWSC Holdings, LLC \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Debt amount		$ 78,900
Operating Segments \| WWSC Holdings, LLC \| Hirschfeld Holdings, LP \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Business acquisition date	Feb. 07, 2018
Purchase price for acquisition	$ 111,300
Cash consideration for acquisition	96,600
Incremental debt acquired	$ 14,700
Operating Segments \| Jazwares, LLC \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Debt amount		41,000
Operating Segments \| Bourn & Koch, Inc. \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Debt amount		14,800
Operating Segments \| Integrated Project Services LLC \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Debt amount		13,300
Operating Segments \| Kentucky Trailer \| Alleghany Capital Corporation Segment
Segment Reporting Information [Line Items]
Debt amount		$ 8,900
Pacific Comp
Segment Reporting Information [Line Items]
Disposal date				Dec. 31, 2017