Summary of Alleghany Capital Product and Service Revenues (Detail) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Other Revenues [Line Items]
Product and service revenues		$ 1,272,902	$ 800,425	$ 2,422,064	$ 1,568,260
Operating Segments
Other Revenues [Line Items]
Product and service revenues		1,268,900	800,300	2,398,300	1,568,100
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Product and service revenues		1,262,300	789,200	2,383,600	1,548,400
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Product and service revenues	[1]	508,600	407,200	970,100	811,700
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Product and service revenues	[2]	$ 753,700	$ 382,000	$ 1,413,500	$ 736,700

[1] For the three and six months ended June 30, 2022 and 2021, the vast majority of industrial product and service revenues were recognized as goods and services transferred to customers over time.

[2] For the three and six months ended June 30, 2022, approximately 55 percent and 57 percent, respectively, of consumer & services product and service revenues were recognized as services transferred to customers over time, with the remainder recognized as goods transferred at a point in time. For the three and six months ended June 30, 2021, approximately 55 percent and 59 percent, respectively, of consumer & services product and service revenues were recognized as services transferred to customers over time, with the remainder recognized as goods transferred at a point in time.