Summary of Alleghany Capital Product and Service Revenues (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Other Revenues [Line Items]
Product and service revenues		$ 3,789,668	$ 2,521,092	$ 2,328,848
Operating Segments
Other Revenues [Line Items]
Product and service revenues		3,775,700	2,512,400	2,316,300
Operating Segments | Alleghany Capital Corporation Segment
Other Revenues [Line Items]
Product and service revenues		3,736,400	2,477,500	2,289,300
Operating Segments | Alleghany Capital Corporation Segment | Industrial Segment
Other Revenues [Line Items]
Product and service revenues	[1]	1,662,300	1,220,900	1,105,600
Operating Segments | Alleghany Capital Corporation Segment | Non-industrial Segment
Other Revenues [Line Items]
Product and service revenues	[2]	2,074,100	1,256,500	1,183,700
Operating Segments | Alleghany Capital Corporation Segment | Corporate & Other
Other Revenues [Line Items]
Product and service revenues		$ 0	$ 100	$ 0

[1] For 2021, 2020 and 2019, the vast majority of industrial product and service revenues were recognized as goods and services transferred to customers over time.

[2] For 2021, 2020 and 2019, approximately 50 percent, 59 percent and 70 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.