Summary of Alleghany Capital Product and Service Revenues (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2022	Mar. 31, 2021
Other Revenues [Line Items]
Product and service revenues		$ 1,149,162	$ 767,835
Operating Segments
Other Revenues [Line Items]
Product and service revenues		1,129,400	767,700
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Product and service revenues		1,121,400	759,100
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Product and service revenues	[1]	461,600	404,400
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Product and service revenues	[2]	659,800	354,700
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Product and service revenues		$ 0	$ 0

[1] For the three months ended March 31, 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 months ended March 31, 2022 and 2021, approximately 59 percent and 63 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.